Preheat the broiler.
Heat the oil in a large skillet over a medium heat. Add the onions, peppers, and salt and saute for 15 minutes, stirring occasionally.
Cut the bread into quarters crosswise and cut each quarter in half, being careful not to cut all the way through, so the 2 haves stay connected. Scoop the soft center out of the bread.
Put the bread, open-faced, on a baking sheet. Place 1 meatball and some sauce on 1 side of each piece of bread. Top with some of the pepper mixture. Moisten the other side of each sandwich with a little sauce and sprinkle with the cheese. Broil until the cheese is melted, about 4 minutes.