Slice zucchini into 1/4-inch rounds. Lay in a single layer on paper towels, sprinkle with salt, and let sweat for 15 minutes. Pat completely dry with fresh towels - this prevents a watery casserole.

Heat a large skillet over medium-high heat. Add ground beef and cook, breaking up with a spoon, until no pink remains, about 6 minutes. Add onion and cook until softened, 4 minutes. Stir in garlic and cook 30 seconds until fragrant.

Add crushed tomatoes, tomato paste, Italian seasoning, and red pepper flakes if using. Simmer for 8 minutes, stirring occasionally, until thickened and flavors meld. Taste and season with salt and black pepper.

In a bowl, combine ricotta, beaten egg, 1/4 cup Parmesan, and 1 cup mozzarella. Stir until smooth.

Preheat oven to 375 degrees F. Spread a thin layer of meat sauce in a 9x13 inch baking dish. Add half the zucchini in an overlapping layer. Spread half the ricotta mixture, then half the remaining meat sauce. Repeat layers once more.

Finish with remaining 1 cup mozzarella and 1/4 cup Parmesan. Cover with foil and bake 25 minutes. Remove foil and bake 10 minutes more until cheese is bubbling and lightly golden. Let rest 10 minutes before garnishing with basil and serving.