Preheat the oven to 350ºF (170ºC). In a medium bowl, mix together the ground beef, bread crumbs, onion, salt and pepper, garlic powder, and half of the mozzarella cheese. Form the mixture into a log, and place it into an 8” x 8” baking dish.  Bake for 50 minutes in the preheated oven, or until the center is no longer pink. Let stand for 5 minutes, then slice into ½” slices. Place a few slices onto each hoagie roll, cover with marinara sauce, and sprinkle remaining mozzarella cheese over. Wrap each sandwich with aluminum foil, and return to the oven for 15 minutes, until bread is lightly toasted and cheese is melted. Let stand 15 minutes before eating. Each sandwich serves 2.