Heat the oven to 400°F. Slice the zucchini lengthwise into strips about ⅛–¼ inch thick, using a mandoline for the most even results.

Arrange the zucchini slices on paper towels or a clean kitchen towel. Sprinkle lightly with salt, let them stand for 10–15 minutes, and pat them very dry.

Place the zucchini on parchment-lined baking sheets and roast for 10–15 minutes, turning once if needed, until softened and some moisture has evaporated. Do not cook the slices until limp.

Heat the olive oil in a large skillet over medium-high heat. Cook the ground beef, Italian sausage, or combination with the onion for 6–8 minutes, breaking up the meat until browned.

Add the garlic and cook for about 30 seconds. Drain excess fat if necessary, then stir in the marinara, Italian seasoning, pepper, optional dried basil or oregano, and red pepper flakes.

Simmer the meat sauce for 5–10 minutes so the flavors blend and the sauce thickens slightly. In a bowl, combine the ricotta, egg, Parmesan, and about ½ cup of the mozzarella.

Reduce the oven temperature to 375°F. Spread a thin layer of meat sauce in a 9-by-13-inch baking dish, then arrange zucchini slices over the sauce.

Add some of the ricotta mixture, more meat sauce, and mozzarella. Repeat for 2–3 layers, finishing with meat sauce and the remaining mozzarella.

Cover the dish loosely with foil and bake for 30 minutes. Remove the foil and bake for another 10–15 minutes, until the lasagna is bubbling and lightly browned.

Let the lasagna rest for 10–15 minutes before slicing. This gives the hot cheese and sauce time to set, making cleaner portions possible.