Heat the third measure of oil in the reserved pan on medium-high heat. Cook meatballs, in batches if needed, for about 5-8 mins, turning frequently, until cooked through. Return all meatballs to the pan along with onion mixture, flour and beef stock. Stir to combine and simmer for 2 mins, until the sauce has thickened (if the sauce is too thick, add a splash of water). Grate over the cheese and cook until melted. Season to taste with salt and pepper.