Nonna's Meatball Sandwich Recipe - Los Angeles Times

Ingredients

• 1 1/2 cups (55 g) soft breadcrumbs, from crusty stale bread (see Note)
• 3/4 cup (180 ml) whole milk
• 1 pound (455 g) ground chuck (80% lean)
• 8 ounces (225 g) sweet Italian pork sausage, casings removed
• 1/2 large yellow onion, grated on the largest holes of a box grater
• 2 garlic cloves, minced
• 1/2 cup (50 g) freshly grated Parmigiano-Reggiano
• 3 tablespoons finely chopped fresh flat-leaf parsley
• 1 large egg, lightly beaten
• 1 1/2 teaspoons fine sea salt
• 1/2 teaspoon freshly ground black pepper
• 3 tablespoons extra-virgin olive oil
• 1/2 large yellow onion, finely chopped
• 1 can (28 ounces / 795 g) crushed tomatoes, preferably San Marzano
• 2 tablespoons coarsely chopped fresh basil
• 1/2 teaspoon sugar
• Salt
• Freshly ground black pepper
• 4 crusty Italian rolls
• 12 slices low-moisture mozzarella cheese