1. In a bowl, combine butter, mustard, horseradish, salt, and pepper.
2. Using a wire whisk beat the mixture until it is creamy and light.
3. Serve butter soft as a spread for bread.
4. Garnish with parsley, if desired.
5. The butter can be served as an accompaniment to roast beef and grilled meats.
6. It goes well with corn on the cob.