• Preheat the oven to 400°F/200°C.

Vegetable Prep:

• Slice vegetables approximately 1/8-inch thick. A mandolin speeds up this process a lot and works well on the squash. Depending on how soft your eggplant and tomatoes are, you can still use a mandolin.,If they are too soft, slice with a sharp knife.
• Use a non-stick spray to coat the inside of a 9 by 13-inch large baking dish. Set aside. I used a deep oval ,casserole which worked well
• In a large skillet, over medium heat, heat 2 tablespoons of olive oil.
• Sauté the sliced onions for about 3-5 minutes or until slightly softened.
• Add the red bell peppers, sauté another 3-5 minutes or until slightly softened. Season with salt and pepper
• Add the garlic and sauté briefly for about 1 minute.

Assemble Gratin:

• Transfer these sautéed vegetables-onions, peppers, and garlic-into a lightly greased baking dish. There's no need to arrange them in any particular order; simply spread them out to create an even bottom layer.
• Arrange the sliced eggplant, zucchini, summer squash and tomatoes in alternating layers in the baking dish.
• Add the oregano or basil, salt and pepper.
• Drizzle the top with the remaining olive oil.
• Place in the oven, uncovered, for 40-45 minutes or until the vegetables are golden brown in color.

Breadcrumb Topping

• Combine breadcrumbs, grated cheese, dried oregano or basil, and salt and pepper to taste.
• Add a pinch of chili flakes or a few pinches of paprika. (optional)
• Sprinkle breadcrumb topping evenly over the top 10 minutes before the end of the bake time