Combine butter, parsley, mustard, and horseradish (and salt if adding) in a bowl. Mix well. Transfer mixture to a condiment bowl and cover with plastic wrap. Refrigerate until ready to use. Allow to come to room temperature to use.
Or roll into a log about 1 and 1/2 inch thick. Twist the ends to close. Refrigerate at least one hour or until firm. Remove from refrigerator and slice into 1/2 inch ‘coins’ when ready to use.