Extract revised for OED Online

    6. mushy peas orig. Eng. regional (north.), a dish or portion of marrowfat peas cooked until soft and served in their own juice.
 
   1973 in R. Clarke Last of Summer Wine Scripts (1976) iv. 75 Clegg: We only left him last night. Stuffing his face with fish, chips and *mushy peas. 1975 C. WRIGHT Yorkshire 4 In south Yorkshire a portion of mushy (‘mooshy’) peas is popular with fish and chips. These are sold in cartons but if made at home dried peas are soaked overnight and boiled until the liquid gets into the pea and it becomes soft. 1981 Guardian Weekly (Nexis) 30 Aug. 19 Mutton stew and mushy peas would have been the sailors' victory feast, had not the ship turned turtle and sunk before the eyes of the king. 1994 Observer 28 Aug. 10/6 Benidorm, once dismissed in Britain as a lager louts' paradise, serving fish and chips and mushy peas, has spruced up its image.