Put butter, peas, lettuce, onion, sliced leeks (to clean leeks, and carrots into saucepan. Cover and simmer for about 30 minutes. Add water, potatoes, sugar, and salt. Cook another 30 minutes.
When done press through food mill. Rinse pan. Put chunk of butter into soup saucepan. Add sorrel and sauté 1 minute. Add puréed soup