In large heavy saucepan, melt butter over low heat. Add lettuce, peas, salt and sugar. Increase heat to moderate, cover tightly and cook, shaking pan until peas start to steam, about 5 minutes. Reduce heat to moderately low and cook until peas are just tender, 6 to 8 minutes.
Cooking fresh peas with lettuce produces a dish that is about as good as it can get.