Step 1 Melt the butter in a large pot. Add the lettuce and cook over low heat 5 minutes.
Step 2 Add the peas, stir and cook uncovered, stirring occasionally, 15 minutes. Add the water, bring to a simmer and continue to simmer, uncovered, 25 minutes.
Step 3Cool slightly. Puree the soup in batches in a food processor or blender.
Step 4Return the soup to the pot, add the salt and pepper and heat through. Ladle into soup plates and serve immediately.