Heat the oil in a large pot and cook the leek and onion until soft, about 5 minutes, stirring periodically.
Add the broth, potatoes, salt and pepper. Bring to a boil, then reduce heat to medium and cook about 30 minutes, until potatoes are soft.
Use an immersion blender to blend the soup until smooth.
!For Hot Soup
Stir in butter and serve with croutons and chervil (or chives).
!For Cold Soup
After blending, stir in the cream and chill. Serve garnished with chives.