1 small onion, grated
1/4 cup chopped fresh Italian parsley leaves
1 large egg
3 tablespoons ketchup
3 garlic cloves, minced
1 teaspoon salt
1/4 teaspoon ground black pepper
2/3 cup grated Parmesan
1/4 cup dried Italian-style bread crumbs
10 ounces ground beef
8 ounces ground pork
1/3 cup extra-virgin olive oil
Marinara sauce, 26-ounce jar (see comments)
12 (3 to 3 1/2-inch diameter) Italian rolls, split, warm
1/3 cup freshly grated Parmesan
3 ounces sliced provolone
To make the meatballs: Whisk the first 7 ingredients in a large bowl to blend. Stir in the cheese and bread crumbs. Mix in the beef, veal, and pork. Using about 2 tablespoons of meat for each, shape the meat mixture into 1 1/4 to 1 1/2-inch-diameter meatballs. Place on a baking sheet.
Heat the oil in heavy large frying pan over medium-high heat. Working in 2 to 3 batches, add the meatballs and saute until browned on all sides, about 5 minutes. Using a slotted spoon, transfer the meatballs to a plate. Pour off any excess oil. Return all the meatballs to the pan. Add the marinara sauce. Simmer over medium-low until the sauce thickens slightly and the flavors blend, about 10 minutes. Season the sauce, to taste, with salt and pepper. (The meatball mixture can be made 1 day ahead. Cool, then cover and refrigerate. Rewarm over medium heat before continuing.)
To make the sandwiches: Spoon the hot meatballs with some sauce over the bottoms of the rolls. Place 1/2 slice provolone on top of meatballs and sprinkle with the Parmesan. Spread more sauce over the roll tops, then fold the tops of the rolls over and serve.
Cook's Note: The sandwiches can be assembled then wrapped individually in foil. Rewarm the sandwiches in the oven at 350 degrees F for about 15 minutes.