Melt the butter in a saucepan, add the peas, lettuce hearts, bouquet garni, salt, sugar and water. Simmer with the lid on for 40 minutes. Remove the lettuce and strain the peas, reserving the liquid. The amount of liquid should be very little but reduce it to about 2 tablespoonfuls, whisk in the cream and pour over peas. Cut lettuce hearts into quarters and place on the peas.