To assemble the meatballs, combine the beef, bread crumbs, milk, salt and pepper and seasoning. Roll into 12 balls and set aside. (I did this step the night prior)
Heat the olive oil in a large pan. Dice the onions and cook until just softened. Add the meatballs and cook until browned on both sides. About two minutes per side.
Add the sauce, cover with a lid and simmer for 20 minutes.
Split the rolls and place a meatball, some sauce and shredded cheese on the bottom half. Replace the top and serve.
Recipe by Friday is Cake Night at https://www.fridaycakenight.com/2014/04/meatball-sliders/