Melt butter with garlic powder in a large skillet.
Cut a slice of the crust off the top and bottom of each hoagie roll. Brush the cut sides with some of the garlic-flavored butter.
Split the rolls and line the insides with mozzarella cheese slices. Mix spaghetti and spaghetti sauce together in a bowl; divide between the hoagie rolls and sandwich the rolls around the cheese and pasta mixture.
Grill the sandwiches in the skillet until the cheese melts completely, about 5 minutes per side.
