Zucchini Summer Squash Tomato Gratin

This gratin is what late-summer dinners are all about: simple vegetables, bold flavor, and a golden, crisp top. Thinly sliced zucchini, yellow squash, and tomatoes bake together until tender, then get finished with a crunchy, cheesy crumb. It’s low-effort, high-reward comfort food that still feels light.

Serve it with grilled chicken, fish, or a crusty slice of bread and you’ve got a complete meal. It also makes a great make-ahead side for any cookout or weeknight table.

Why This Recipe Works

• Salt-and-rest method: Briefly salting the squash pulls out excess moisture, helping the gratin bake up creamy, not watery.
• Layered flavor: Garlic, herbs, and a quick olive oil drizzle season every slice, so the dish tastes balanced from top to bottom.
• Two-texture topping: Parmesan melts into the vegetables while buttery breadcrumbs toast into a crisp crust.
• High heat roast: Baking at 400–425°F concentrates flavors and gives you caramelized edges without overcooking the veg.

Ingredients

• 2 medium zucchinis, thinly sliced (about 1/8 inch)
• 2 medium yellow summer squash, thinly sliced
• 3 medium ripe tomatoes, thinly sliced
• 1 small yellow onion, very thinly sliced (optional but recommended)
• 3 cloves garlic, minced
• 3 tablespoons extra-virgin olive oil, plus more for drizzling
• 1/2 cup freshly grated Parmesan or Pecorino Romano
• 3/4 cup fresh breadcrumbs (or panko)
• 2 tablespoons unsalted butter, melted
• 1 teaspoon fresh thyme leaves (or 1/2 teaspoon dried)
• 2 tablespoons chopped fresh basil (plus more for serving)
• 1/2 teaspoon crushed red pepper flakes (optional)
• Kosher salt and freshly ground black pepper
• 1 teaspoon lemon zest (optional, for brightness)

How to Make It

1. Prep the pan: Heat the oven to 425°F (220°C). Lightly oil a 9×13-inch baking dish or similar casserole.
2. Slice and salt: Lay zucchini and yellow squash slices on a sheet pan.Sprinkle with 1 teaspoon kosher salt. Let sit 10–15 minutes to draw out water. Pat dry thoroughly.
3. Season the veg: In a large bowl, toss squash, tomatoes, and onion with 3 tablespoons olive oil, garlic, thyme, red pepper flakes (if using), 1/2 teaspoon salt, and plenty of black pepper.
4. Layer: Arrange the slices upright in slightly overlapping rows alternate zucchini, squash, and tomato for color.Tuck onion slices throughout. Drizzle any leftover seasoned oil from the bowl over the top.
5. Par-bake: Cover loosely with foil and bake for 20 minutes until the vegetables start to soften.
6. Make the topping: Stir breadcrumbs with Parmesan, melted butter, lemon zest, a pinch of salt, and black pepper until evenly moistened.
7. Top and finish: Remove foil. Sprinkle the crumb mixture evenly over the vegetables.Bake uncovered for 15–20 minutes, until the top is deep golden and the veg are tender with lightly caramelized edges.
8. Rest and serve: Let the gratin rest 10 minutes. Sprinkle with fresh basil. Taste and finish with a little more salt, pepper, or a light olive oil drizzle if needed.

Keeping It Fresh

• Storage: Cool completely, then cover and refrigerate for up to 3 days.
• Reheat: Warm in a 375°F oven for 10–15 minutes.Add a spoonful of olive oil if the topping looks dry.
• Make-ahead: Assemble, cover, and refrigerate up to 6 hours. Add the breadcrumb topping right before baking so it stays crisp.
• Freeze? Not ideal. The tomatoes and squash release water after thawing, softening the texture.

Why This is Good for You

• Fiber-rich veggies: Zucchini, squash, and tomatoes support digestion and keep you satisfied.
• Antioxidants: Tomatoes bring lycopene; basil and thyme add polyphenols that support overall wellness.
• Lighter fats: Olive oil adds heart-healthy monounsaturated fats without weighing the dish down.
• Balanced comfort: A small amount of cheese and crumbs gives you flavor and crunch without heavy cream.

What Not to Do

• Don’t skip salting the squash. That quick step prevents a watery, soggy gratin.
• Don’t use unripe tomatoes. They’ll taste bland and release less sweetness.
• Don’t overcrowd the pan. If your dish is too small, use two pans so the vegetables roast instead of steam.
• Don’t burn the topping. If it browns too fast, tent with foil for the last few minutes.

Recipe Variations

• Mediterranean twist: Add olives, capers, and a sprinkle of oregano; finish with crumbled feta.
• Cheesy comfort:</-strong> Swap half the Parmesan for shredded mozzarella or Gruyère.
• Gluten-free: Use gluten-free panko or crushed gluten-free crackers.
• Protein boost: Scatter cooked white beans or chickpeas between layers before baking.
• Herb swap: Try rosemary or dill in place of thyme and basil for a different vibe.
• Crunch upgrade: Mix chopped toasted almonds or pine nuts into the crumb topping.

FAQ

Can I slice the vegetables thicker?

You can, but aim for no thicker than 1/4 inch. Thicker slices take longer to cook and can make the topping brown before the center is tender.

What if my tomatoes are very juicy?

Use slightly firmer tomatoes, or lightly seed them. You can also add an extra tablespoon of breadcrumbs under the first layer to absorb juices.

Do I have to use onion?

No, but it adds sweetness and depth. If you skip it, consider adding a pinch of sugar or an extra clove of garlic to balance the acidity.

Can I make this dairy-free?

Yes. Omit the cheese and use dairy-free butter or extra olive oil in the crumbs. Add nutritional yeast for a savory, cheesy note.

How do I know it’s done?

The vegetables should be tender when pierced with a knife, edges lightly caramelized, and the top deep golden and crisp. If the center still feels firm, give it 5 more minutes.

Final Thoughts

This zucchini summer squash tomato gratin is proof that simple ingredients can taste like something special. With a few smart steps and a hot oven, you get tender layers and a crunchy top every time. Keep it as a side or make it the star with a salad and bread. Either way, it’s a bright, easy bake you’ll come back to all season.