Mustard Chive Sauce

Directions

1. 1
   In a small bowl stir together the mustard, the vinegar, the sugar, the salt, and 3 tablespoons water, making a smooth paste, and let the mixture stand, covered, for 10 minutes.
2. 2
   Transfer the mixture to the top of a double boiler set over barely simmering water and whisk in the butter until it is just melted and incorporated thoroughly.
3. 3
   Remove the pan from the heat and stir in the chives.
4. 4
   Transfer the sauce to a sauce boat and serve it warm or at room temperature.