Peel and dice the potatoes. Cover with water in a saucepan and bring to a boil. Add salt and simmer for about 15 minutes until the potatoes are soft. Pour into a sieve.
Process the boiled potatoes into a puree with a potato press. This makes the puree fine and fluffy. Alternatively, you can use a potato masher. However using this the consistency might not be quite as evenly creamy.
Mix the mashed potatoes in a saucepan with butter and lemon juice. Stir in hot water until the desired consistency is achieved. Season to taste with salt and, if necessary, with horseradish cream. Serve hot.