Preheat the oven on the broiler setting at 400 degrees. In a microwave safe dish heat the meatballs and marinara sauce.
Slice your roll lengthwise. Spoon on the meatballs and cover with any remaining marinara sauce. Arrange the sliced cheese on top (I cut my piece in half to make it cover a longer/narrower surface area… kinda like the cheese “triangles” at sub shops).
Place the sandwich, open faced, on a cookie sheet and place under the broiler (make sure the rack is positioned so there is a few inches between the top of the sandwich and the heating element). Broil until the cheese is melted and the edges of the roll is brown and crispy.