Sloppy Joe Meatball Bake

Name: Sloppy Joe Meatball Bake
Ingredients: 1 1/2 lb. lean ground beef; 1 large egg; 3/4 c. panko bread crumbs; 1/3 c. whole milk; 1 medium yellow onion, finely chopped, divided; 1 tbsp. plus 1 tsp. Worcestershire sauce, divided; Kosher salt; Freshly ground black pepper; 3 tbsp. extra-virgin olive oil, divided; 1 medium red bell pepper, seeds and ribs removed, finely chopped; 3 cloves garlic, minced; 1 (15-oz.) can tomato sauce; 1/2 c. ketchup; 1/4 c. water; 1 tbsp. apple cider vinegar; 1 tbsp. packed light brown sugar; 1 tsp. yellow mustard; 1 1/2 c. (6 oz.) shredded mozzarella