1. Wash the head of lettuce briefly under cold running water, remove the tough or blemished outer leaves, and cut the lettuce lengthwise into quarters. To shred the lettuce, cut out the core and slice each of the quarters crosswise into 1/8-inch-wide strips.
2. In a heavy 12-inch skillet, melt the 4 tablespoons of butter bits over moderate heat. When the foam begins to subside, add the onions and turn them with a spoon until they are coated with butter.
3. Drop in the lettuce and toss the shreds about to moisten them evenly. Then add the green peas, sugar, chervil, salt and water and, stirring constantly, bring to a boil over high heat.
4. Reduce the heat to low, cover the skillet tightly and simmer until the peas and onions are tender but still intact. This may take anywhere from 8 to 15 minutes, depending on the size and age of the green peas and the onions.
5. Blend the teaspoon of softened butter and the flour together in a bowl, add them to the simmering vegetables and stir over moderate heat until the liquid comes to a boil and thickens slightly.
6. Taste for seasoning and serve the "green peas a la francaise" at once from a heated bowl.
