Cut a thin slice off the top of each roll; scoop out bread from inside. Crumble 1-1/4 cups of the bread and place in a large bowl. Cover rolls and tops with plastic wrap; set aside. To the crumbled bread, add beef, egg, milk, diced onion, salt and pepper. Shape into 27 meatballs, about 1-1/2 inches each. In a large skillet, cook meatballs in 1 tablespoon oil for 20 to 25 minutes or until no longer pink. Remove with a slotted spoon; set aside. Add remaining oil to skillet; sauté green peppers and sliced onion until tender. Remove with a slotted spoon; set aside. Stir flour into skillet. Add chili sauce and water; bring to a boil. Cook and stir for 1 to 2 minutes. Stir in brown sugar and mustard. Add meatballs, peppers and onion; cover and simmer for 20 minutes. Meanwhile, warm rolls in a 325 °F oven for 8 to 10 minutes. Spoon three meatballs and sauce into each roll; replace tops.