Cut celery and potato into large cubes. Cover vegetables in a saucepan with milk, simmer until tender and strain then. Puree vegetable. Stir in cream and bring to a boil. Stir in cold butter into small pieces. Season with pepper, salt, nutmeg and lemon juice.
Fry liver briefly in butter on both sides. Liver has been still rosy inside. Season with salt, pepper and vinegar.