Green Peas with Shredded Lettuces

Put a pound and a half of fresh green peas, recently shelled, into a saucepan with two tender lettuce heads shredded up coarsely, one small onion, a bunch of parsley, salt, a pinch of sugar and sufficient cold water to reach to about their height; cover the saucepan and cook for twenty to twenty-five minutes on a hot fire. Suppress the parsley and onion, thicken the liquid with a small piece of butter kneaded with flour (No. 579), and finish off the fire with a lump of fresh butter divided in small pats.