1 tablespoon olive oil, plus more for frying
1/2 cup finely chopped onion
2 cloves garlic, minced
3 slices white bread, crusts removed, torn in pieces
1/3 cups milk
3/4 pounds ground beef chuck
1/2 pounds ground pork
1/4 pounds ground veal
1/2 cups freshly grated Parmesan or Pecorino Romano cheese
1 large egg, beaten
2 tablespoons chopped fresh basil or Italian parsley
1/2 teaspoons kosher salt
1/2 teaspoons freshly ground black pepper
3 cups homemade or store-bought marinara sauce
1 1/2 cups baby arugula
1 package (15-ounce) party-size potato rolls, split, or other mini soft rolls
10 slices (thin) provolone cheese, halved
In a small saucepan, heat oil over medium-low heat and sauté onions and garlic until translucent, about 5 minutes; let cool. In a medium bowl, combine bread and milk; let stand until mushy. Add ground meats, onion mixture, cheese, egg, basil, salt, and pepper; mix with hands until just combined (don’t overmix). Using a small ice cream scoop or a tablespoon, form into 20 small meatballs.
Heat about 1/4 cup of olive oil in a large, deep skillet over medium heat. Working in batches, fry meatballs until brown all over, about 7 minutes per batch. Transfer to a plate. Discard drippings and wipe skillet. Add marinara sauce to skillet and return meatballs to sauce. Bring to a boil, reduce heat to low, cover with lid slightly ajar, and simmer, turning meatballs occasionally, about 30 minutes or until meatballs are tender.
To serve, place arugula leaves on bottom of each roll. Top each with 1 meatball. Drizzle each meatball with some of the sauce and top with a cheese slice and the top of the roll.