Pulse the diced bread in a food processor until you have an even mix of fine and coarse crumbs. Add 1/4 cup of the Parmesan, the parsley leaves, 1 1/2 teaspoons of the minced garlic, 1/4 teaspoon of the salt, and 1/8 teaspoon of the pepper, and process until evenly mixed. Reserve 2 tablespoons of the breadcrumb mixture for the filling. Mix 1 1/2 tablespoons of the extra-virgin olive oil into the remaining breadcrumbs, transfer the mixture to a small container, and set it aside. (Alternatively, you can use store-bought fine fresh crumbs: Combine 3/4 cup breadcrumbs with 1 tablespoon minced parsley, 1/2 teaspoon minced garlic, 1/4 cup finely grated Parmesan, 3/4 teaspoon salt, and 1/8 teaspoon freshly ground black pepper. Set aside 2 tablespoons, and mix 1 1/2 tablespoons extra-virgin olive oil into the remainder.)

Lay the zucchini on a flat work surface, and using a sharp knife, slice off the top quarter of each squash lengthwise. Next, slice a sliver off the bottom of each squash to help keep it stable. Using a small melon baller or spoon, remove the inner flesh from the zucchini to form a small boat shape, leaving a shell that is approximately 1/4 inch thick. Cut the zucchini pulp into 1/4-inch dice, and reserve it separately. Lightly salt the inside of the zucchini shells with the remaining 1/4 teaspoon salt. Set them, hollow side down, on paper towels to drain while you prepare the filling.

Heat 1 tablespoon of the olive oil in a 12-inch skillet. Add the sausage and sauté until it is golden, using a spoon to break it into small pieces, about 6 minutes. Add the onions and cook until they are soft, 3 to 4 minutes. Add the chopped zucchini and cook for 2 minutes. Add the tomatoes and the remaining 1 1/2 teaspoons garlic, and cook, stirring, until the moisture has evaporated and the filling comes together, 2 minutes. Remove the skillet from the heat, stir in the reserved 2 tablespoons breadcrumb mixture, and season with additional salt and pepper if necessary.

Preheat the oven to 350°F.

Rub the outside of the zucchini with the remaining 1/2 tablespoon olive oil, and season them lightly with salt and pepper. Turn the zucchini hollow side up, and lightly pat the insides with paper towels. Using a tablespoon or other small spoon, fill the zucchini with the warm filling. Top with the reserved breadcrumbs. Lay the zucchini in a baking dish, and bake for 30 minutes, or until golden brown and crispy on top.