Melt half the butter in a wide, shallow casserole that can be used for serving. Add the onions, and cook gently for 2 to 3 minutes, until tender but not browned. Add the quartered lettuce hearts and turn them over once or twice in the butter. Add the peas, hot chicken stock, and some salt and pepper, and simmer rapidly for 3 to 4 minutes, turning the lettuce hearts now and then, until the vegetables have started to soften and about three-fourths of the liquid has evaporated. Put the pieces of ling on top of the vegetables, then cover and simmer for 7 to 8 minutes, until the fish is cooked.