Peel the horseradish root and grate until you have 1 cup. If you have a cuisinart with a grating attachment, it will be much easier. But it’s do-able by hand.
Combine the grated horseradish, vinegar, salt, and sour cream and blend well. At this point it will be creamy but a little watery. If you’d like it to be thicker, add the mayonnaise.
Let sit in the refrigerator for a couple of hours at least before serving.
Lasts up to a week in a sealed container in the fridge.