The delicate flavors of zucchini and yellow squash intensify when baked with layers of Parmesan cheese, tomatoes, and caramelized onions. Unlike most gratins which have a rich white sauce, I use Parmesan cheese and a layer of caramelized onions and garlic to make this dish creamy and flavorful. Tastes delectable with some chunks of crusty bread to sop up the luscious juices. This gratin is easy to make, and can be assembled up to 6 hours in advance, covered, and kept at room temperature until time to be baked. —Kiss the Cook
6 large plum tomatoes, sliced in 1/4 inch slices
2 zucchinis, sliced in 1/4 inch slices
2 yellow squash, sliced in 1/4 inch slices
1 tablespoon extra-virgin olive oil plus more for oiling baking dish
Preheat the oven to 375 degrees. Coat a large baking dish with olive oil.
In a large skillet, heat extra-virgin olive oil over medium heat. Add the onions, sea salt and black pepper and cook until the onions are very soft and caramelized, about 15 minutes. Add the garlic and sauté for 1 minute more.
Layer the dish with half of the tomato slices and then all of the yellow squash slices. Sprinkle with half the grated Parmesan and all of the fresh basil. Pour the caramelized onion mixture over in a even layer. Then, add the zucchini slices and the final layer of tomatoes. Top with remaining Parmesan cheese.
Cover with foil and bake for 20 minutes. Remove foil and broil for 3 minutes or caramelized and until crispy on top. This gratin is equally good warm or at room temperature, making it a perfect do-ahead dish.