In a large skillet, heat oil over medium heat.
Add onion and bell pepper; cook, stirring frequently for 6 minutes.
Add pasta sauce, Make-Ahead Meatballs, and ½ cup water.
Bring to a boil, reduce heat, and simmer for 20 minutes.
Pre-heat broiler and line a rimmed baking sheet with aluminum foil.
Split bread pieces lengthwise, being careful not to cut all the way through.
Spoon 3 meatballs and sauce into each bread piece; top with cheese.
Place on prepared pan.
Broil 4 to 5 minutes or until cheese is melted; serve immediately.