01 -
In a large bowl, combine ground beef, egg, panko breadcrumbs, whole milk, 15 g of the chopped onion, 1 teaspoon Worcestershire sauce, kosher salt, and black pepper. Mix until just combined. Scoop the mixture and roll into approximately 33 meatballs (about 4 cm each) and place them on a plate.
02 -
Preheat the oven to broil on high. In a large Dutch oven or oven-safe pot over medium-high heat, heat 30 ml olive oil. Arrange the meatballs in a single layer and sear, turning occasionally, until browned on all sides, 7–9 minutes. Transfer meatballs to a plate and discard excess grease from the pot.
03 -
Return the pot to medium heat and add bell pepper, garlic, the remaining onion, and 15 ml olive oil. Sauté, stirring occasionally, until vegetables are tender, 8–10 minutes. Stir in tomato sauce, ketchup, water, apple cider vinegar, brown sugar, yellow mustard, and the remaining 15 ml Worcestershire sauce. Scrape any browned bits from the bottom. Return the meatballs to the pot and simmer, stirring occasionally, until the sauce thickens slightly and the meatballs are fully cooked, 4–6 minutes. Remove from heat and evenly distribute mozzarella cheese over the meatballs.
04 -
Broil the skillet until the cheese is bubbly and golden, about 2 minutes. Serve hot directly from the skillet or in sandwich rolls as desired.