Mustard sauce has the same goal as a North Carolina vinegar sauce—to cut the fattiness of the pork. The mustard and sugar make this thicker and saucier than one made with vinegar (you could dip your bread in it or lick the spoon, for example—something you wouldn’t do with a vinegar sauce).
Step 1: Melt the butter in a heavy nonreactive saucepan over medium heat. Add the onion and garlic and cook until soft but not brown, about 3 minutes.
Step 2: Stir in the mustard, brown sugar, vinegar, and hot sauce and add 1/2
cup of water. Let the sauce simmer, uncovered, until thick and richly flavored, 6 to 10 minutes. Taste for seasoning, adding more hot sauce as necessary and seasoning with salt and pepper to taste. Let the sauce cool to room temperature before serving. In the unlikely case you have any mustard sauce left, store it in a clean jar in the refrigerator. It will keep for at least a week; bring it to room temperature before using.
You could use a different sweetener, substituting honey or molasses for the brown sugar. Use 3/4 of a cup of honey. If substituting molasses, start with 3/4 of a cup, but taste for sweetness—you may need to add a little more, since molasses is not as sweet as brown sugar.