4 leeks, white part,sliced thin
1 medium onion, sliced thin
2 tbsp butter
4 medium potatoes, diced
3 cups chicken broth
1 tsp salt
1 1/2 cups milk
1 1/2 cups cream 18%
salt
pepper

Stir fry in butter the white part of the leeks and the onion until golden.
Add potatoes, broth and salt. Boil for 35 minutes, until potatoes are well cooked.
Strain through a sieve or pass through a blender, replace on the fire and add milk and cream.
Stir contiuously. As soon as it starts boiling remove from fire. Let cool.
Add salt and pepper to taste ( it may take more salt than usual because when cold the soup would otherwise taste too bland).
Strain through a sieve and refrigerate.
Serve with a sprinkling of chopped chives or with some watercress, as a decoration.