Preheat oven to 350 degrees F (175 degrees C).
Trim stems from zucchini and slice lengthwise. Scoop out the seeds and put in a bowl. Mix seeds with sausage, garlic, bread crumbs, and Parmesan cheese. Stuff squash with sausage mixture and place in a 9x13 inch baking pan. Pour spaghetti sauce over the top and cover the pan with foil.
Bake in preheated oven until sausage is browned and cooked through, about 45 minutes. Remove foil and cover with mozzarella cheese. Return to the oven and cook until cheese is melted, about 15 minutes more.
