Preheat the oven to 400F.
In large skillet - combine ground beef, onions, bell peppers, salt and pepper. Sauté until beef is browned and vegetables are tender. Drain off any excess grease from the skillet or using a slotted spoon, remove the meat mixture from the skillet to a large bowl.
Place sub rolls (open side up) into casserole dish. Divide meat mixture evenly between 6 sliced sub rolls.
Thinly slice cheese then place on top of the meat filled sub rolls.
Bake for 5 minutes until cheese is melted and sub rolls have browned.
Top with your favorite sauce such as: ketchup, BBQ sauce, mayonnaise, or sriracha mayo.