Put the horseradish in a saucepan with water to cover, add salt, and bring to a boil. Simmer for about 1 hour, or until the horseradish is very tender, like a boiled potato. Drain, reserving some of the cooking liquid.
Puree the horseradish in a blender or food processor with the crème fraîche or sour cream and as much of the cooking liquid as you need to allow