Preheat oven to 325˚F (165˚C). Grease or line a rimmed baking sheet with parchment.
In a large bowl, add the breadcrumbs, milk, Parmesan, onion, garlic cloves, dried parsley, salt, pepper, eggs, beef, and pork.
Massage the mixture just until all the ingredients are evenly combined, then divide into 10 equal portions, about 5 ounces (142 grams) each.
In a medium bowl, toss together the diced capocollo, salami, ham, mozzarella, and provolone.
Make a well in the center of one of the meatball portions, then add a heaping spoonful of the diced meat and cheese filling.
Press together the outside edges of the meatball over the filling to seal it completely inside, then gently roll into a ball and place on the prepared baking sheet.
Repeat with remaining meatball portions and filling, arranging them at least 2 inches (5 centimeters) apart.
Bake in the preheated oven until the meatballs are golden and heated completely through, 45 minutes to 1 hour.
Serve hot over spaghetti or on their own.