This is a quick and delicate summer soup that can be served hot or cold. Later in the year, the lettuce could be replaced with leaves from three big bunches of watercress.
Place the peas, lettuce (or watercress if using), and potato in a medium-size pan. Season generously with salt, add the hot stock and simmer for 4 minutes or until the peas are tender. Pour the soup into the bowl of a food processor or blender and liquidise it. Pour it through a sieve into a clean pan and reheat it gently.