Source: A modified Martha Stewart recipe
1/4 cup well-drained horseradish
1/2 cup plus 1 heaping tablespoon sour cream
2 tablespoons of Dijon mustard
1 tablespoon of whole grain mustard.
Mix well and chill.
My notes: This sauce is easy and very tasty. The original recipe calls for 1 1/2 cups of sour cream - so much sour cream that the other tastes do not shine through. This sauce is a heavily influenced by the mustard, so if you're looking for horseradish to dominate, this may not be the sauce for you.