01 -
Combine ground beef, egg, panko bread crumbs, milk, 40 g onion, 5 ml Worcestershire sauce, kosher salt, and black pepper in a large bowl. Mix until just combined. Roll mixture into approximately 33 balls, each about 4 cm in diameter (2 tablespoons per ball), and arrange on a plate.
02 -
Preheat oven to broil on high. In a large Dutch oven or ovenproof pot over medium-high heat, warm 15 ml olive oil. Arrange meatballs in a single layer and cook, turning to brown on all sides, for 7 to 9 minutes. Transfer meatballs to a plate and discard excess fat from the pot.
03 -
Return pot to medium heat and add remaining olive oil, bell pepper, garlic, and 100 g chopped onion. Cook, stirring occasionally, until vegetables are softened, 8 to 10 minutes. Add tomato sauce, ketchup, water, apple cider vinegar, brown sugar, yellow mustard, and remaining Worcestershire sauce. Stir, scraping up any browned bits from the bottom of the pot.
04 -
Return meatballs to the pot. Stir gently to coat with sauce and bring to a simmer. Cook, stirring occasionally, until sauce thickens slightly and meatballs are fully cooked, 4 to 6 minutes. Remove from heat and sprinkle evenly with shredded mozzarella.
05 -
Place pot under broiler and cook until cheese is bubbling and golden, about 2 minutes. Serve hot, garnished as desired.