Summer Vegetable Gratin

1. Adjust oven rack to upper-middle position and heat oven to 400 degrees. Brush 13 by 9-inch baking dish with 1 tablespoon oil and set aside. Line 2 baking sheets with triple layer of paper towels and set aside.
2. Toss zucchini and summer squash slices with 1 teaspoon salt in large bowl; transfer to colander. Let sit until zucchini and squash release at least 3 tablespoons of liquid, about 45 minutes. Arrange slices on 1 prepared baking sheet and cover with another triple layer of paper towels. Firmly press each slice to remove as much liquid as possible.
3. Place tomato slices in single layer on second prepared baking sheet, sprinkle evenly with 1⁄2 teaspoon salt, and let sit 30 minutes. Cover with double layer of paper towels and press firmly to dry tomatoes.
4. Meanwhile, heat 1 tablespoon oil in 12-inch nonstick skillet over medium heat until shimmering. Add onions, remaining 1⁄2 teaspoon salt, and 1⁄4 teaspoon pepper. Cook, stirring occasionally, until onions are softened and dark golden brown, 20 to 25 minutes. Set onions aside.
5. Combine garlic, 3 tablespoons oil, remaining 1⁄2 teaspoon pepper, and thyme in small bowl. In large bowl, toss zucchini and summer squash in half of oil mixture, then arrange in prepared baking dish. Arrange caramelized onions in even layer over squash. Slightly overlap tomato slices in single layer on top of onions. Spoon remaining garlic-oil mixture evenly over tomatoes. Bake until vegetables are tender and tomatoes are starting to brown on edges, 40 to 45 minutes.
6. Meanwhile, process bread in food processor until finely ground, about 10 seconds. (You should have about 1 cup crumbs.) Combine bread crumbs, remaining 1 tablespoon oil, Parmesan, and shallots in medium bowl. Remove baking dish from oven and increase heat to 450 degrees. Sprinkle bread-crumb mixture evenly on top of tomatoes. Bake gratin until bubbling and cheese is lightly browned, 5 to 10 minutes. Sprinkle with basil and let sit for 10 minutes before serving.

SUMMER VEGETABLE GRATIN WITH ROASTED PEPPERS AND SMOKED MOZZARELLA
You can use store-bought roasted red peppers or make your own . If using store-bought, rinse and pat the peppers dry before using.
Substitute 4 ounces shredded smoked mozzarella (1 cup) for Parmesan and 3 roasted red peppers, cut into 1-inch pieces, for summer squash (do not salt roasted peppers).

ASSEMBLING VEGETABLE GRATIN
1. Toss salted zucchini and squash in half of garlic-oil mixture, then arrange in prepared greased baking dish.
2. Spread caramelized onions in even layer on top of zucchini and squash.
3. Slightly overlap salted tomatoes in single layer on top of onions, then top with remaining garlic-oil mixture.
4. When vegetables are tender, sprinkle gratin with bread-crumb mixture, then continue to bake until golden brown.