1. Preheat broiler to high.
2. Combine first 6 ingredients in a bowl; mix gently. Working with damp hands, gently shape beef mixture into 16 meatballs. Arrange meatballs in a single layer on a heavy-duty baking sheet coated with cooking spray, leaving 6 inches of open space on one end. Place bell pepper halves, skin sides up, on open space on pan. Broil 6 inches from heat for 7 minutes, turning meatballs once. Remove pan from oven. Place peppers in a small paper bag; fold to close tightly. Let stand for 10 minutes. Remove peppers from bag. Peel and slice into 1/4-inch-thick strips.
3. Bring marinara sauce to a simmer in a large skillet over medium-low heat; add meatballs, tossing to coat. Keep warm.
4. Heat a grill pan over medium-high heat; coat pan with cooking spray. Add onion slices; cook 4 minutes on each side or until charred. Remove onion from pan.
5. Coat insides of buns with cooking spray, and broil 2 minutes or until toasted. Top each bun with 4 meatballs and 1/3 cup sauce. Divide peppers, onions, and cheese evenly among sandwiches; broil sandwiches 2 minutes or until bubbly and browned. Garnish with basil, if desired.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.