Spread the onion evenly onto the bottom of the slow cooker dish.
In a large bowl, mix the ground meat with bread crumbs, egg, seasonings, and Parmesan cheese.
Use your hands to roll the mixture into golf-sized balls (about 2 tablespoons per meatball) and layer them evenly inside the slow cooker dish. Pour the can of tomatoes and pasta sauce over the meatballs.
Cover the dish with the lid and cook on low for 6 hours or high for 3 hours.
To serve, slice or fold open each hoagie roll, sprinkle a little cheese at the bottom, and top with meatballs and some sauce. Sprinkle additional cheese and enjoy.