Add the onions and 1 1/2 cups water to a saucepan. Bring the water to a boil over high heat and cook the onions for 20 minutes. Add the butter, Worcestershire sauce, white wine vinegar and mustard flour. Reduce the heat to medium low and cook until the sauce thickens, about 45 minutes.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Snow's BBQ, Lexington, Texas