Cut zucchini in half lengthwise and using a spoon, scoop out flesh, leaving ½-in. shells. Chop the scooped out flesh of the zucchini in small pieces and set aside.
Brown sausage in a large sauté pan over medium high heat. Add onion and cook until meat is no longer pink and onion is tender. Add garlic and cook for another minute. If necessary, drain any excess fat.
Stir in tomatoes, basil, and chopped zucchini. Add salt and pepper to taste.
In a small bowl, combine breadcrumbs and parmesan cheese.
Divide the sausage mixture between the shells (boats), and then top with breadcrumb mixture.
Bake for 20-25 minutes or until zucchini is tender and top is golden brown.