Shred the lettuce and other leaves finely. Cut the cucumber into match-like strips. Melt the butter in a deep pan and fry the vegetables very gently for 3 min. Add the stock, boiling, to the vegetables and simmer for 10 min. Blend the flour with half the milk, stir this into the soup and cook till the flour thickens. Season very carefully, add lemon juice to taste and cook slightly. Mix the remaining milk with the egg yolks and cream, add these to the cooked soup and cook till the egg yolks thicken, but do not allow to boil.
Serve at once.