With a teaspoon, scoop out the flesh from the interior of the zucchini, so they resemble boats. Place the zucchini boats in a 9-by-13-inch casserole dish. Chop the zucchini flesh and set aside.
In a medium saute pan over medium-high heat, add the olive oil and let heat through. Add the sweet and the hot Italian sausage, and cook for 4 minutes. Add the garlic, onion, chopped zucchini flesh, tomatoes and some salt and pepper. Cook until softened, about 4 minutes.
In a medium bowl, add the Parmesan, mozzarella, breadcrumbs and parsley; mix to combine.
Spoon in and mound the sausage mixture into the zucchini boats. Sprinkle the breadcrumb mixture over top. Place in oven and bake until golden on top, 20 minutes.