Zucchini and Summer Squash Gratin with Parmesan and Fresh Thyme

There are some dishes that, no matter how many times I make them, transport me back to sun-drenched afternoons in my grandmother’s garden. This Zucchini and Summer Squash Gratin is one of them. I remember the gentle weight of ripe squash in my hands, the vibrant green and sunny yellow hues, and the aroma of fresh thyme wafting from her kitchen window. It wasn’t just a meal; it was a celebration of the season’s bounty, a simple yet profound expression of love served in a humble baking dish. This gratin, with its layers of tender vegetables, fragrant herbs, and nutty Parmesan, has become a cherished staple, a comforting reminder of simpler times and the exquisite pleasure of homegrown goodness.

Recipe Overview

• Prep Time: 25 minutes
• Cook Time: 65-70 minutes
• Total Time: 1 hour 30 minutes – 1 hour 35 minutes
• Servings: 4-8 (4 as a main dish, 6-8 as a side)
• Yield: 1 shallow gratin dish
• Dietary Type: Vegetarian

Ingredients

This gratin showcases the best of summer produce, elevated by the savory depth of Parmesan and the bright, herbaceous notes of fresh thyme.

For the Onions:

• 2 tablespoons olive oil
• 2 medium onions, thinly sliced
• 2 garlic cloves, minced

To Assemble the Gratin:

• 1 1/4 lbs ripe tomatoes, cored and cut into 1/4 inch slices
• 3/4 lb zucchini, cut into 1/4 inch slices on the bias
• 3/4 lb other green summer squash, cut into 1/4 inch slices on the bias
• 3/4 lb summer squash, cut into 1/4 inch slices on the bias
• 3/4 lb golden zucchini, cut into 1/4 inch slices on the bias
• 3 tablespoons olive oil (divided)
• 1/4 cup fresh thyme leaves (divided)
• 1 teaspoon coarse salt (divided)
• 1 1/4 cups freshly grated Parmigiano-Reggiano cheese (Note: While this recipe calls for Parmigiano-Reggiano, a good quality mozzarella can be used if Parmigiano-Reggiano is unavailable, as noted in some culinary experiences.)
• Fresh ground black pepper

Equipment Needed

• Medium skillet
• 2-quart shallow gratin dish (preferably oval)
• Shallow plate
• Medium bowl
• Measuring cups and spoons
• Sharp knife
• Cutting board

Instructions

Crafting this gratin is a meditative process, much like tending a summer garden. Each step builds upon the last, transforming simple ingredients into a symphony of flavors and textures.

1. Sauté the Onions: Begin by preparing the aromatic base for your gratin. In a medium skillet, heat 2 tablespoons of olive oil over medium heat. Add the thinly sliced onions and sauté, stirring frequently, until they become limp and golden brown. This process can take about 20 minutes. It’s important to keep a close eye on them and reduce the heat to medium-low if they start to brown too quickly, to ensure they soften and caramelize beautifully without burning. Once the onions are tender and golden, add the minced garlic cloves and sauté for an additional 1 to 2 minutes until they are soft and fragrant.

2. Prepare the Gratin Dish: Spread the cooked onions and garlic evenly across the bottom of an oiled 2-quart shallow gratin dish. A slight sheen of oil on the dish will prevent sticking and contribute to the gratin’s succulence. Allow this layer to cool slightly while you prepare the vegetables.

3. Preheat the Oven: Heat your oven to 375°F (190°C). This moderate temperature is perfect for slowly coaxing the flavors out of the vegetables and achieving a beautifully tender result.

4. Drain the Tomatoes: Place the 1 1/4 lbs of ripe tomato slices on a shallow plate. Allow them to sit for a few minutes to drain any excess liquid. Discard these collected juices, as too much moisture can make the gratin watery.

5. Toss the Squash: In a medium bowl, combine the zucchini slices, other green summer squash slices, summer squash slices, and golden zucchini slices. Add 1 1/2 tablespoons of the olive oil, 2 tablespoons of the fresh thyme leaves, and 1/2 teaspoon of the coarse salt. Gently toss everything together to ensure the vegetables are lightly coated.

6. Layer the Gratin: Now comes the artful assembly. Reserve half of the grated Parmigiano-Reggiano cheese for the topping. Sprinkle 1 tablespoon of the fresh thyme over the cooled onions and garlic in the gratin dish. Begin layering your vegetables at one end of the baking dish. Lay a row of slightly overlapping tomato slices across the width of the dish and sprinkle with a little of the reserved cheese. Next, add a row of zucchini slices, overlapping the tomatoes by two-thirds. Sprinkle this layer with cheese. Follow this with a row of summer squash, again sprinkling with cheese. Continue this pattern, alternating between tomato, zucchini, and squash rows, sprinkling each layer with cheese, until the gratin dish is full and all your vegetables are used. This layering technique creates beautiful visual appeal and ensures even cooking.

7. Season and Drizzle: Season the top lightly with fresh ground black pepper and the remaining 1/2 teaspoon of coarse salt. Drizzle the remaining 1 1/2 tablespoons of olive oil evenly over the entire surface of the gratin.

8. Add the Topping: Combine the reserved half of the cheese with the remaining 1 tablespoon of fresh thyme leaves. Sprinkle this flavorful mixture evenly over the entire gratin.

9. Bake the Gratin: Place the prepared gratin dish in the preheated oven. Cook for 65 to 70 minutes, or until the gratin is well-browned all over and the juices have bubbled for a while and reduced considerably. The aroma filling your kitchen will be an indicator of its readiness – a rich, savory perfume.

10. Rest Before Serving: This is a crucial step for a perfectly cohesive gratin. Let the gratin cool for at least 15 minutes before serving. This resting period allows the flavors to meld and the structure of the gratin to set, preventing it from falling apart when sliced.

Expert Tips & Tricks

To elevate your Zucchini and Summer Squash Gratin from simply delicious to truly exceptional, consider these professional insights:

• Uniform Slices are Key: For the most even cooking, aim for consistently thin, 1/4-inch slices of all your vegetables. A mandoline slicer can be an invaluable tool here, but a sharp knife and a steady hand will also do the trick.
• Don’t Skimp on the Onions: Those caramelized onions are the soul of the base, providing a deep, sweet foundation. Be patient during this sautéing step; it’s worth the time.
• The Power of Resting: That 15-minute rest might seem long when your kitchen smells so inviting, but it’s essential for the gratin to firm up. If you cut into it too soon, it will be a delicious, but potentially messy, pile of vegetables.
• The Cheese Variation: While Parmigiano-Reggiano offers a distinctly nutty and salty profile, don’t hesitate to experiment with a blend. A touch of Gruyère or even a sharp white cheddar can add another layer of complexity. And as noted, if you find yourself out of Parmigiano, a good quality mozzarella will still yield a wonderfully cheesy result.
• Herbal Freshness: Thyme is a classic pairing, but feel free to introduce other complementary herbs. A sprinkle of finely chopped rosemary or oregano can add a lovely Mediterranean twist.

Serving & Storage Suggestions

This Zucchini and Summer Squash Gratin is incredibly versatile. It shines as a stunning vegetarian main course, especially when served with some crusty bread to sop up the luscious juices. It also makes a magnificent side dish to grilled chicken, roasted fish, or lamb. For an elegant presentation, let it rest in the dish and serve directly from it, scooping generous portions onto warm plates.

Storage: Leftovers can be stored in an airtight container in the refrigerator for up to 3 days. To reheat, gently warm individual portions in the oven at 325°F (160°C) until heated through, or in the microwave for a quicker, though perhaps slightly less crisp, result. Freezing is not generally recommended as the texture of the cooked vegetables can become mushy upon thawing.

Nutritional Information

The following nutritional information is an estimate and can vary based on specific ingredients and portion sizes.

Nutrient	Amount per Serving (approx.)	% Daily Value (approx.)
Calories	350-450 kcal	18-23%
Total Fat	25-35 g	32-45%
Saturated Fat	8-12 g	40-60%
Cholesterol	15-25 mg	5-8%
Sodium	800-1200 mg	35-52%
Total Carbohydrate	15-25 g	5-9%
Dietary Fiber	3-5 g	11-18%
Sugars	5-8 g	N/A
Protein	10-15 g	20-30%

Note: Daily Values are based on a 2,000-calorie diet. Your daily values may be higher or lower depending on your calorie needs.

Variations & Substitutions

The beauty of this gratin lies in its adaptability.

• For a Gluten-Free Version: This recipe is naturally gluten-free, making it a safe and delicious option for those avoiding gluten.
• Adding a Protein Punch: For a heartier dish, consider adding a layer of cooked, crumbled Italian sausage or shredded cooked chicken between the vegetable layers.
• A Touch of Spice: If you enjoy a little heat, a pinch of red pepper flakes added to the onion sauté or sprinkled over the top before baking can add a delightful kick.
• Different Cheeses: While Parmigiano-Reggiano is traditional, feel free to experiment. A mix of Gruyère and mozzarella, or even a sharp cheddar, can create a different flavor profile. Just ensure you’re using cheeses that melt well.
• Seasonal Adaptations: While this gratin sings with zucchini and summer squash, in other seasons, you could adapt the concept. Think of thinly sliced potatoes and leeks in the fall, or root vegetables like parsnips and carrots in the winter, though baking times may need adjustment.

FAQs

Q: Why does the recipe instruct to let the gratin set out before serving?
A: Letting the gratin rest allows the cooked vegetables and cheese to firm up, creating a more cohesive and sliceable dish, rather than a loose collection of ingredients.

Q: Can I use dried thyme instead of fresh?
A: While fresh thyme offers a brighter, more vibrant flavor, you can substitute dried thyme. Use about 1 teaspoon of dried thyme for every 1 tablespoon of fresh. Add it during the onion sautéing phase to allow its flavor to bloom.

Q: My gratin looks a bit watery. What did I do wrong?
A: Overcrowding the dish with vegetables or not draining the tomatoes sufficiently can lead to excess moisture. Ensure your slices are not too thick and drain those tomatoes well. The extended baking time is also designed to reduce juices.

Q: How do I know when the gratin is truly “well-browned all over”?
A: Look for a rich, golden-brown crust on top and along the edges. The vegetables should be tender when pierced with a fork, and the juices bubbling vigorously at the bottom of the dish.

Q: Can I prepare this gratin ahead of time?
A: You can assemble the gratin a few hours in advance and keep it covered in the refrigerator. Allow it to sit at room temperature for about 30 minutes before baking, and you may need to add a few extra minutes to the baking time to ensure it’s thoroughly heated through.

Final Thoughts

This Zucchini and Summer Squash Gratin is more than just a recipe; it’s an invitation to savor the simple pleasures of the season. It’s a dish that embodies warmth, comfort, and the delightful essence of home cooking. I encourage you to embrace the process, enjoy the aromas that fill your kitchen, and share this beautiful creation with loved ones. Serve it with a crisp, dry white wine like a Sauvignon Blanc or a light-bodied Pinot Noir, and don’t forget that crusty bread—it’s the perfect partner for sopping up every last drop of that rich, savory sauce. I’m eager to hear about your experience with this timeless summer classic!