Quick Recipe Version (TL;DR)
Quick Ingredients
- 2 pounds zucchini, trimmed and sliced into 1/4-inch rounds
- 1 1/2 pounds ripe tomatoes, sliced into 1/4-inch rounds
- 1 small yellow onion, thinly sliced
- 1 1/4 teaspoons fine sea salt, divided
- 1/2 teaspoon freshly ground black pepper
- 4 tablespoons plus 1 teaspoon extra-virgin olive oil, divided
- 4 garlic cloves, finely minced
- 1 teaspoon chopped fresh thyme
- 1 teaspoon chopped fresh oregano
- 1/4 teaspoon crushed red pepper flakes, optional
- 3/4 cup panko breadcrumbs
- 2/3 cup finely grated Parmesan cheese, divided
- 1/4 cup fresh basil leaves, torn or thinly sliced
Do This
- 1. Heat the oven to 400°F and lightly oil a 9-by-13-inch baking dish.
- 2. Salt the zucchini and tomatoes for 10 minutes, then pat very dry.
- 3. Stir together 3 tablespoons olive oil, garlic, thyme, oregano, pepper, red pepper flakes, and remaining salt.
- 4. Layer zucchini, tomato, and onion in the baking dish, drizzling with garlic herb oil and sprinkling with half the Parmesan.
- 5. Cover with foil and bake for 20 minutes.
- 6. Mix panko with remaining Parmesan and 1 tablespoon olive oil, sprinkle over the gratin, and bake uncovered for 25 minutes.
- 7. Rest for 10 minutes, finish with fresh basil, and serve warm.
Why You’ll Love This Recipe
- It tastes like peak summer: Juicy tomatoes, tender zucchini, garlic, basil, and olive oil bake together into a savory, golden gratin.
- Simple ingredients, big payoff: A handful of pantry staples turns everyday vegetables into a bubbling, Parmesan-topped dish.
- Flexible and useful: Serve it as a side with grilled chicken, fish, pasta, or grains, or enjoy it as a light vegetarian dinner with a green salad.
- Great texture: Salting and drying the vegetables keeps the gratin flavorful, not watery, while the panko-Parmesan topping bakes crisp and golden.
Grocery List
- Produce: 2 pounds zucchini, 1 1/2 pounds ripe tomatoes, 1 small yellow onion, 4 garlic cloves, fresh basil, fresh thyme, fresh oregano
- Dairy: Parmesan cheese
- Pantry: Extra-virgin olive oil, panko breadcrumbs, fine sea salt, black pepper, crushed red pepper flakes
Full Ingredients
For the Vegetables
- 1 teaspoon extra-virgin olive oil, for greasing the baking dish
- 2 pounds zucchini, about 4 medium, trimmed and sliced into 1/4-inch rounds
- 1 1/2 pounds ripe but firm tomatoes, about 4 to 5 medium, sliced into 1/4-inch rounds
- 1 small yellow onion, about 6 ounces, very thinly sliced
- 1 teaspoon fine sea salt, divided for salting the zucchini and tomatoes
For the Garlic Herb Oil
- 3 tablespoons extra-virgin olive oil
- 4 garlic cloves, finely minced
- 1 teaspoon chopped fresh thyme
- 1 teaspoon chopped fresh oregano
- 1/2 teaspoon freshly ground black pepper
- 1/4 teaspoon fine sea salt
- 1/4 teaspoon crushed red pepper flakes, optional
For the Parmesan Breadcrumb Topping
- 3/4 cup panko breadcrumbs
- 2/3 cup finely grated Parmesan cheese, divided
- 1 tablespoon extra-virgin olive oil
For Finishing
- 1/4 cup fresh basil leaves, torn or thinly sliced
- Additional freshly ground black pepper, to taste
Step-by-Step Instructions
Step 1: Heat the oven and prepare the dish
Place a rack in the center of the oven and heat to 400°F. Lightly coat a 9-by-13-inch baking dish or a shallow 2 1/2- to 3-quart gratin dish with 1 teaspoon extra-virgin olive oil. A shallow dish is best because it gives the vegetables plenty of surface area and helps the topping brown beautifully.
Step 2: Slice and salt the vegetables
Slice the zucchini and tomatoes into even 1/4-inch rounds. Spread the zucchini slices on a clean kitchen towel or a few layers of paper towels and sprinkle with 3/4 teaspoon fine sea salt. Place the tomato slices on a separate towel-lined surface and sprinkle with the remaining 1/4 teaspoon fine sea salt. Let them sit for 10 minutes. This quick salting step draws out excess moisture, which helps the gratin bake up tender and flavorful rather than watery.
After 10 minutes, gently blot the zucchini and tomatoes dry on both sides. You do not need to rinse them; just pat away the beads of moisture and any excess salt on the surface.
Step 3: Make the garlic herb oil
In a small bowl, stir together 3 tablespoons extra-virgin olive oil, the minced garlic, thyme, oregano, black pepper, 1/4 teaspoon fine sea salt, and crushed red pepper flakes if using. This mixture will season the vegetables as they bake and create that fragrant, savory base that makes the gratin taste rich without needing cream.
Step 4: Layer the zucchini, tomatoes, and onion
Arrange the zucchini, tomato, and onion in the prepared baking dish, overlapping the slices slightly. You can make neat rows, diagonal stripes, or a rustic shingled pattern. As you layer, drizzle small spoonfuls of the garlic herb oil over the vegetables so the seasoning is distributed throughout the dish.
Sprinkle 1/3 cup of the Parmesan cheese evenly over the layered vegetables. Try to tuck some cheese between the slices as well as over the top; it will melt into the juices and help bind the gratin together.
Step 5: Bake covered until the vegetables begin to soften
Cover the baking dish tightly with aluminum foil and bake for 20 minutes at 400°F. This covered bake gives the zucchini, tomatoes, and onions time to soften and release their juices before the breadcrumb topping is added. It also prevents the crumbs from over-browning too early.
Step 6: Add the Parmesan breadcrumb topping
While the vegetables bake, combine the panko breadcrumbs, the remaining 1/3 cup Parmesan cheese, and 1 tablespoon extra-virgin olive oil in a small bowl. Stir until the crumbs are lightly coated and the cheese is evenly mixed in.
After the covered baking time, carefully remove the foil. The vegetables should look glossy and partially tender, with some juices bubbling around the edges. Sprinkle the Parmesan breadcrumb mixture evenly over the top, making sure to cover the center and corners.
Step 7: Bake uncovered until bubbling and golden
Return the dish to the oven, uncovered, and bake for 25 minutes, until the vegetables are fully tender, the juices are bubbling around the edges, and the topping is crisp and golden brown. If the top browns quickly before the vegetables are tender, loosely tent the dish with foil for the final few minutes.
Step 8: Rest, garnish, and serve
Let the gratin rest for 10 minutes before serving. This short rest allows the juices to settle so each spoonful holds together better. Scatter the fresh basil over the top and finish with a little more black pepper, if desired. Serve warm as a side dish, or make it a light vegetarian dinner with crusty bread, white beans, farro, pasta, or a simple green salad.
Pro Tips
- Use ripe but firm tomatoes: Very soft tomatoes can release too much liquid and fall apart during baking. Roma, vine-ripened, heirloom, or beefsteak tomatoes all work if they are sliceable.
- Do not skip the salting step: Zucchini has a high water content. Salting and blotting it first gives you a more concentrated flavor and a less watery gratin.
- Keep slices the same thickness: Aim for 1/4-inch slices so the vegetables cook evenly and layer neatly.
- Add the crumbs halfway through: Baking the vegetables first, then adding the panko topping, keeps the breadcrumbs crisp instead of soggy.
- Finish with basil after baking: Fresh basil tastes brightest when added at the end rather than baked for the full time.
Variations
- Mediterranean-style gratin: Add 1/3 cup crumbled feta, 1/4 cup chopped Kalamata olives, and a pinch of dried marjoram along with the Parmesan.
- Heartier vegetarian dinner: Scatter 1 1/2 cups drained and rinsed cannellini beans into the baking dish before adding the topping, or serve the gratin over cooked farro, couscous, or creamy polenta.
- Extra cheesy version: Add 4 ounces thinly sliced fresh mozzarella between some of the vegetable layers, then bake as directed. The gratin will be softer and more melty, so allow a full 10-minute rest before serving.
Storage & Make-Ahead
Store leftover zucchini and tomato gratin in an airtight container in the refrigerator for up to 4 days. Reheat individual portions in the microwave, or reheat the baking dish, covered with foil, in a 350°F oven for 15 to 20 minutes, removing the foil for the last 5 minutes to help the topping re-crisp. The breadcrumbs will soften slightly after storage, but the flavor remains excellent.
To make ahead, slice and salt the vegetables up to 6 hours in advance, pat them dry, and refrigerate them in separate covered containers. You can also mix the garlic herb oil and the breadcrumb topping ahead of time; store the oil at room temperature for up to 2 hours or in the refrigerator for up to 1 day, and keep the breadcrumb topping covered at room temperature. Assemble and bake just before serving for the best texture. Freezing is not recommended because zucchini and tomatoes become watery after thawing.
Nutrition (per serving)
Calories: 215 kcal | Carbs: 17g | Protein: 7g | Fat: 14g | Saturated Fat: 3g | Fiber: 4g | Sugar: 8g | Sodium: 620mg | Cholesterol: 8mg
