How to Make It
Arrange pork shoulder and belly in an even layer on a rimmed baking sheet. Freeze grinder attachments, 3/16-inch grinder plate, and pork 30 minutes.
Fit a meat grinder with chilled grinder attachments and grinder plate. Position a bowl under grinder. Set meat grinder to medium speed, and grind chilled meat through feed tube into bowl. Place ground meat in refrigerator until ready to use.
Heat 2 tablespoons oil in a large Dutch oven over medium. Add onions and garlic; cook, stirring often, until very tender, 10 to 12 minutes. Remove from heat. Reserve 1 cup onion mixture; chill at least 30 minutes or until ready to use.
Return remaining onion mixture in Dutch oven to heat over medium. Add ’nduja and remaining 1 tablespoon oil; cook, stirring with a wooden spoon to break ’nduja into small pieces. Add tomato passata, 1/2 cup water, and 1/2 teaspoon salt; cook, stirring occasionally, until vigorous bubbles break surface of sauce. Reduce heat to low to maintain a gentle bubble; cook, stirring occasionally, until flavors meld, about 30 minutes. Remove from heat; set aside.
While tomato sauce cooks, place bread and milk in a large bowl. Using a rubber spatula, press down on bread to moisten completely. Let stand 10 minutes. Mash mixture until thick and creamy. Stir in cheese, flour, egg yolks, pepper, reserved onion mixture, and remaining 1 tablespoon plus 1/2 teaspoon salt.
Add chilled ground meat to bread mixture. Using your hands, mix thoroughly until mixture is combined and feels slightly sticky. Shape mixture evenly into 24 (about 2-inch) balls. Arrange meatballs on a rimmed baking sheet. Chill 30 minutes.
Preheat oven to broil with rack about 8 inches from heat source. Broil meatballs in preheated oven until tops are lightly browned, about 10 minutes, rotating baking sheet 180 degrees on rack halfway through cook time. While meatballs broil, return tomato sauce to a gentle bubble over medium-low.
Transfer broiled meatballs to tomato sauce using a spatula; discard drippings on baking sheet. Cook meatballs over medium-low, gently stirring occasionally, until a thermometer inserted in thickest portion of meatballs registers 160°F, about 10 minutes. Remove from heat; let stand 15 minutes. Season with salt to taste. Divide meatballs and sauce evenly among plates; top with additional cheese.