1. Remove the wilted outer leaves of the lettuce and trim the stem. Rinse the lettuce in cold water, spreading the leaves apart gently, to remove all traces of sand. Cut the lettuce into 4 or 6 wedges, and bind each wedge with soft string to keep it in shape while cooking.
2. In a heavy 3-quart saucepan, bring the peas, lettuce wedges, onions, parsley, 6 tablespoons butter, water, salt and sugar to a boil over moderate heat, toss lightly to mix flavors, then cover the pan tightly and cook for 30 minutes, stirring occasionally, until the peas and onions are tender and the liquid nearly cooked away.
3. If the liquid hasn't evaporated, cook the peas uncovered, shaking the pan constantly, for a minute or two until it does.
4. Remove the parsley and cut the strings off the lettuce. Gently stir in 2 tablespoons of soft butter; taste and season. Transfer to a heated vegetable dish and serve in small bowls. It is traditional to eat petits pois with a spoon.
