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, 3 minutes.
Step 2: Stir in the mustard, brown sugar, vinegar, water, and hot sauce. Simmer the sauce, uncovered, until thick and richly flavored, 6 to 10 minutes. Correct the seasoning, adding salt and pepper to taste. Note: In the unlikely case you have any mustard sauce left, store it in a clean jar in the refrigerator. It will keep for several weeks.