Heat the butter in a saucepan, add the spring onions and fry for 2–3 minutes until softened. Add the peas, cook for 2 minutes then shred and add the lettuce. Pour over the stock, add the sugar and a little salt and pepper and bring to the boil.
Cover and simmer gently for 10 minutes until the lettuce is wilted but still bright green. Allow to cool slightly, then purée in batches in a blender or food processor until smooth. Stir in the cream, then taste and adjust the seasoning if needed. Chill well.
Ladle the soup into small bowls set on plates. Serve with Little Gem lettuce leaves topped with a small amount of crab mixed with the mayonnaise and lemon juice and sprinkled with paprika.