Stew celery, onion, and potato gently in the butter in a covered pan for 10 minutes. Don't let the vegetables brown. Add stock or water and 1/2 teaspoon of dill weed. Simmer for 20 minutes if you have a blender, 40 minutes if you use a food mill.

Blend or purée the soup. Pour through a strainer into a clean pan (to remove the last few threads of celery), adding a little milk if too thick. Bring slowly to just under the boil, seasoning with salt, pepper and more dill weed if required.

Put the cream into the soup dish, and pour the soup in on top. Swirl round with the ladle before serving, to mix in the cream.