Prepare the tomato sauce by browning sausage links and chopped onion in a large pot with olive oil.

Add tomato puree, garlic, parsley, basil, salt, pepper, and grated Parmesan cheese to the pot.

Simmer the sauce on low heat, stirring regularly, for 1 to 1 1/2 hours.

While the sauce simmers, prepare the zucchini by slicing it.

Beat eggs in a bowl.

Dip the zucchini slices into the beaten eggs, then coat with bread crumbs.

Fry the zucchini slices until golden brown (optional, can also be baked).

Grate the mozzarella and Locatelli or Romano cheese.

In a baking dish, layer the tomato sauce, zucchini slices, mozzarella cheese, and Locatelli or Romano cheese.

Repeat the layers until all ingredients are used, finishing with a layer of cheese on top.

Bake in a preheated oven at 375°F (190°C) for 20-25 minutes, or until the cheese is melted and bubbly.