Prep the zucchini “noodles”: Preheat oven to 400°F (200°C). Slice zucchini lengthwise into 1/8–1/4-inch planks using a sharp knife or mandoline. Lay slices on two lined baking sheets. Sprinkle lightly with salt and let sit 10 minutes to draw out moisture. Pat dry.

Par-bake the zucchini: Brush or spray slices with olive oil. Bake for 10–12 minutes to reduce moisture and soften slightly. Remove and set aside. Lower oven to 375°F (190°C).

Cook the meat sauce: Heat 1 tablespoon olive oil in a large skillet over medium. Add onion and cook 3–4 minutes until softened. Stir in garlic for 30 seconds. Add ground beef (and sausage, if using). Cook, breaking up meat, until browned. Drain excess fat if needed.

Season and simmer: Stir in marinara or crushed tomatoes, tomato paste (if using), Italian seasoning, red pepper flakes, 1/2 teaspoon salt, and pepper. Simmer 8–10 minutes until slightly thickened. Taste and adjust seasoning.

Mix the ricotta layer: In a bowl, combine ricotta, egg, Parmesan, a pinch of salt, and black pepper. Stir until smooth.

Assemble the casserole: Lightly grease a 9x13-inch baking dish. Spread a thin layer of meat sauce on the bottom. Add a layer of zucchini slices, slightly overlapping. Dollop and spread half the ricotta mixture, then sprinkle with one-third of the mozzarella. Repeat layers: sauce, zucchini, remaining ricotta, another third of mozzarella. Finish with the remaining sauce and the final layer of mozzarella.

Bake: Cover loosely with foil (tent so cheese doesn’t stick). Bake at 375°F for 20 minutes. Uncover and bake another 10–15 minutes, until bubbly and golden on top.

Rest and serve: Let the casserole rest 10–15 minutes to set. Garnish with chopped basil or parsley. Slice and serve warm.