1) Preheat the grill.
2) Heat the oil in a large frying pan over a medium heat. Add the onions, peppers, and salt and saute for 15 minutes, stirring occasionally.
3) Cut the bread into quarters crosswise and cut each quarter in half, being careful not to cut all the way through, so the 2 haLves stay connected. Scoop the soft center out of the bread.
4) Put the bread, cut side up, on a baking tray. Place one meatball and some sauce on one 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.
5) Grill until the cheese is melted, about 4 minutes.
6) Serve immediately.