Meatball Sandwich

Name: Meatball Sandwich
Ingredients: 2 medium onions, peeled and minced; olive oil; 2 cloves garlic, peeled and minced; 4 cups (1kg) canned crushed or diced tomatoes, along with their juice; 1 tablespoon tomato paste; 1 1/2 teaspoons salt; 1 1/2 teaspoons dried oregano; 1/2 teaspoon crushed red pepper flakes; one bay leaf; 1/2 teaspoon sugar; 1 large onion, peeled and minced; 1 pound (450g) ground beef (not too lean); 1 pound (450g) ground pork; 3 large eggs; 1/2 cup (25g) grated Parmesan, Asiago, or Pecorino cheese; 1/2 cup (15g) chopped flat-leaf parsley; 1 to 2 tablespoons chopped fresh herbs; 2 teaspoons salt; freshly ground black pepper; 1 1/2 teaspoon whole fennel seeds; 2 cups (220g) dried breadcrumbs; 3/4 cup (180ml) milk; Additional salt and black pepper, for seasoning the onions; Mozzarella or provolone cheese; Crusty Italian or French bread