Choose small, tender summer squash and meaty Roma tomatoes for this Provençale-style dish. Be sure to let this set for a bit after baking. In fact, the flavors are even better at room temperature, making it a great addition to any summertime outdoor buffet or picnic.
- 4 tablespoons extra-virgin olive oil, plus more for the pan
- 1 large sweet onion (about 14 ounces), such as Vidalia, thinly sliced
- Kosher or sea salt and freshly ground black pepper
- 2 cloves garlic, minced
- 1 tablespoon chopped fresh marjoram (or substitute fresh thyme)
- 2 pounds summer squash, preferably a mix of zucchini and yellow, sliced into ¼-inch rounds
- 1¼ pounds ripe, meaty tomatoes, such as Roma, cored and cut into ?-inch-thick slices
- ½ cup freshly grated parmesan
Preheat the oven to 375°F. Lightly oil a large gratin dish or 9- by 14-inch baking dish.
Heat 2 tablespoons of the oil in a medium skillet over medium heat. Add the onion, season with salt and pepper, and cook, stirring frequently, until tender and beginning to brown, about 10 minutes. Add the garlic and 2 teaspoons of the marjoram, and cook until fragrant, another minute. Turn off the heat and set aside.
Place the squash in a large bowl. Drizzle with 1 tablespoon of the oil, season with salt and pepper, and toss to coat.
Spread half of the cooked onion in the bottom of the baking dish. Layer half the squash on top of the onions, arranging them in a shingled pattern in as even a layer as possible. Top with half of the tomato slices. Repeat with another layer of onions, squash, and tomatoes. Season the vegetables with the remaining teaspoon of marjoram and salt and pepper. Drizzle over the remaining tablespoon of olive oil.
Cover with foil, and bake for 30 minutes. Remove the foil, scatter the parmesan over the top, and bake, uncovered, until the vegetables are tender and the top is lightly browned, another 45 minutes. Let the gratin sit for at least 20 minutes before serving. Serve warm or at room temperature.