Wash and cut up the lettuces, also cut up the cucumber and onion, put them into a stewpan, together with 1/2 pint of peas, the mint, and butter. Cover with about 1 quart of cold water, bring to the boil, and simmer gently for 3 hours. Then strain off the liquid and pass the vegetables through a sieve. Add them to the liquid again, and set on the fire. Season and add 1/2 pint green peas previously boiled.