Cook and stir the ground beef in a skillet over medium heat until the meat is browned, breaking the beef up into crumbles as it cooks, about 10 minutes. Drain off excess fat, and transfer the beef into a mixing bowl.
Slice the zucchini in half the long way, and use a spoon to scoop out the flesh, leaving a 1/2-inch thick shell all around the zucchini.
Chop the scooped-out zucchini flesh, and add to mixing bowl. Stir in the bread crumbs, garlic, spaghetti sauce, black olives, and Parmesan cheese; mix the stuffing well.
Lightly stuff both halves of the zucchini with the meat mixture. Place the zucchini halves into a baking dish, and cover tightly with foil.
Bake in the preheated oven for 45 minutes, remove from oven, and remove the foil.
Sprinkle 1/2 cup of shredded mozzarella cheese over each zucchini half.
Move an oven rack to within 6 inches of the oven broiler’s heat source, turn on the broiler, and broil the zucchini until the mozzarella cheese is browned and bubbling, about 5 minutes.