Put the celeriac, sprouts and ham in a deep saucepan with the milk and water or stock and bring to the boil. Simmer gently for 25 minutes or until the celeriac is cooked. Purée in a food processor then return to the pan. Add the sherry and seasoning to taste. If needed, thin with a little more water. Serve alone or sprinkled with sunflower seeds or parsley.