MUSTARD BUTTER (beurre de moutarde, for kidneys, liver, steaks, broiled fish and sauce enrichments)
½ cup butter
1 to 2 tablespoons prepared mustard, the strong Dijon type
salt and pepper to taste
Optional: 2 tablespoons fresh minced parsley or mixed green herbs
1. Cream the butter well. A half-teaspoon at a time, beat in the mustard. Beat in seasonings and optional parsley or mixed herbs to taste.
from Julia Child, Mastering the Art of French Cooking, Volume I