Dice bacon and brown in skillet; remove bacon and drain on paper towel. Reserve grease and slightly saute onions until soft and translucent.
Cut zucchini in half lengthwise. Heat on microwave safe plate, cut side down, for about 3-4 minutes to soften. Using kitchen towel to protect your hands, remove zucchini pulp using a spoon. Add pulp, and garlic to onions and cook a few more minutes. Stir in parmesan cheese, bacon and ground pepper, and remove from heat.
Place zucchini in pan. Cover the bottom of each with a small amount of mozzarella. Fill shells with hot filling. Melt butter in small bowl in microwave. Add breadcrumbs. Top each zucchini with about 1 tsp of crumbs.
Bake for about 35-45 minutes until zucchini are tender, filling is bubbly, and breadcrumbs are browned.