Meatballs coated in marinara sauce, smothered with cheese, and sandwiched in brioche rolls are a party-perfect nibble. For tender meatballs, combine the ingredients gently and don't pack the meat too tightly.
Make the meatballs: Soak bread in milk for 5 minutes. Gently mix bread and milk with meats, Pecorino Romano, parsley, garlic, egg, and 1 teaspoon salt. Refrigerate for at least 30 minutes.
Roll mixture into twenty-four 1-ounce balls (each about the size of a golf ball).
Heat sauce in a large heavy pot over medium heat. Drop meatballs into sauce, and bring to a gentle simmer. Cook, partially covered, until meatballs are cooked through, 20 to 25 minutes.
Make the sandwiches: Preheat oven to 400 degrees. Working in batches, arrange rolls, cut side up, on a baking sheet. Place 1 meatball, a spoonful of sauce, 1 tablespoon mozzarella, and 1 teaspoon Pecorino Romano on bottom halves. Bake until cheese melts, about 3 minutes. Sandwich with basil.
Simmered meatballs can be refrigerated in sauce for up to 3 days. Reheat meatballs in sauce, and assemble sandwiches just before serving.