Combine the ground beef, ground pork, egg, small chopped onion, minced garlic, parmesan, bread crumbs, salt, and pepper in a large bowl, mixing thoroughly. Shape the mixture into 1-inch meatballs and arrange them on a plate. Warm a little oil in a non-stick skillet over medium heat, then sear the meatballs for about 5-6 minutes, turning them midway. Transfer the cooked meatballs to a plate. Return the pan to medium heat and add olive oil, the large chopped onion, and minced garlic, cooking until aromatic. Pour in the tomato sauce, water, Italian seasoning, and salt, bringing the mixture to a gentle simmer. Return the meatballs to the sauce and cook until fully done. Preheat the oven to 200°C/400°F. Halve the hoagie rolls and generously fill each with meatballs and sauce, then top with mozzarella slices. Arrange the open sandwiches on a baking tray and broil for 2-3 minutes until the cheese melts. Serve without delay.