For her vegetable Parmesan casserole, Giada De Laurentiis layers grilled and sauteed vegetables with creamy bechamel sauce, sprinkles on a mixture of mozzarella and Parmesan cheese, then bakes the casserole until the top is golden and forms a crust.
For the sauce: In a 2-quart saucepan, melt the butter over medium heat. Add the flour and whisk until smooth, about 2 minutes. Gradually add the milk, whisking constantly to prevent lumps. Add the lemon zest. Simmer over medium heat, whisking constantly, until the sauce thickens, about 10 minutes.
Place a grill pan over medium-high heat or preheat a gas or charcoal grill. Place an oven rack in the center of the oven. Preheat the oven to 375 degrees F. Butter an 13-by-9-inch glass baking dish.
Toss the zucchini slices, onion slices and peppers with the 1/4 cup of olive oil. Season with salt and pepper. Grill the vegetables for 3 to 4 minutes each side until softened. (Cook's Note: The vegetables can also be baked in a 375 degrees F oven for 15 to 20 minutes until softened.)
Spoon 3/4 cup of the cream sauce over the bottom of the prepared baking dish. Arrange the zucchini slices on top. Sprinkle with 1 cup of mozzarella cheese and 1/3 cup Parmesan cheese. Arrange the peppers in a single layer on top. Spoon 3/4 cup of cream sauce sauce over the peppers. Sprinkle with 1 cup of mozzarella cheese and 1/3 cup Parmesan cheese. Place the onion on top and cover with the remaining sauce. Sprinkle the remaining cheeses on top. Sprinkle the breadcrumbs over the cheese and drizzle liberally with oil. Bake for 30 to 35 minutes until the top is golden and forms a crust.
Cool for 10 minutes before serving.