Soak the stale rolls in water for 2-3 minutes. Preheat the oven to 200°C (392°F). In a large bowl, combine the minced meat, squeezed-out rolls, 2 chopped onions, mustard, 3 eggs, salt, pepper, and 1 minced garlic clove. Mix vigorously.
Grease a baking dish with 1 tablespoon margarine. Shape the meat mixture into golf-ball-sized meatballs and place them in the baking dish.
Wash and cut the red pepper into small strips. Chop the remaining onion and 2 remaining garlic cloves. Heat the remaining margarine in a pan over medium heat. Add the pepper, onion, and garlic and fry for 10 minutes, stirring occasionally, until lightly browned.
Stir in the tomato pieces, salt, pepper, and Italian herbs. In a small bowl, dissolve the cornstarch in the cream, then add to the pan and mix well. Season to taste. Pour the sauce over the meatballs and top with the sliced mozzarella.
Bake on the middle rack at 200°C (392°F) for 30-40 minutes until cooked through (internal temperature: 63°C/145°F).