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.
Cook:
Cover the dish with the lid and cook on low for 6 hours or high for 3 hours.
Serve:
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.