Take the lettuce, wash and drain and chop fine with the onion, put in a saucepan with two heaping tablespoonfuls of butter, cook for about ten minutes, stirring all the time, then add the rice and milk. Let it boil for twenty minutes until the rice is perfectly tender, remove from the fire and press through a purée sieve, using a small potato masher, then strain and press again through a fine hair sieve; this will make it smooth. Season with salt to taste and the cayenne pepper, and the sugar. Put in a fresh saucepan, rub together two heaping teaspoonfuls of butter and the cornstarch and stir into the soup. Let it come to the boiling point and remove from the fire, adding at the last moment the whipped cream. Serve with or without fried croutons.