Combine bread slices, milk and egg in a large bowl; let sit for 5 minutes to allow bread to absorb all the liquid. Break up bread with a spoon until there are no large pieces. Add beef, pork, onions, 1/4 cup Parmesan, parsley, oregano, fennel, salt and pepper. Mix until just combined; form into 12 meatballs.

Heat a layer of olive oil in a deep skillet over medium heat. Working in batches so not to overcrowd the pan, fry meatballs until golden brown on all sides, 7 to 10 minutes. Remove meatballs to a paper towel-lined plate. Remove excess oil from the skillet. Pour in wine or beef broth and scrape the bottom of the pan with a wooden spoon. Stir in tomatoes, garlic, basil, salt and pepper. Add meatballs back to the pan and bring to a gentle simmer. Cover and cook until cooked through, about 20 minutes.

Preheat broiler. Cut baguette pieces in half and drizzle with olive oil. Broil on a foil-lined baking sheet until golden. Place two meatballs on one half of each sandwich. Sprinkle each with mozzarella and remaining Parmesan. Return to the oven and broil until cheese is just melted, 2 to 3 minutes. Serve hot with sauce on the side for dipping.