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 8x8 inch baking dish.
Bake for 50 minutes or until the center is no longer pink. Let stand for 5 minutes, then slice into 1/2 inch slices. Place a few slices onto each hoagie roll, cover with marinara sauce, and sprinkle remaining mozzarella cheese over.
Wrap each sandwich with aluminuim foil, and return to oven for 15 minutes, until bread is lightly toasted and cheese is melted. Let stand 15 minutes befor eating. Each sandwich serves 2.