The gravy poured over these meatballs is called 'pea wet' because it's traditionally the cooking liquid drained straight off a pan of mushy peas.
Minced pork offal, liver and bread are seasoned, formed into balls, wrapped in caul fat and baked until dark and savoury, then served with mushy peas and a splash of the peas' own cooking liquid over the top. Black Country market stalls and chip shops sell them as a filling, cheap plate.