Make a layer of sliced onion on the bottom of oven-proof pot.Rub the dry mustard over entire surface of meat.Mix up the rest of the ingredients in big measuring cup and spread it over the roast.Now, place the roast atop the onion layer in pot.Let stand several hours. When you are ready to cook,preheat oven to hottest (450 or 500 degrees) and roast 15 minutes. Then turn back oven to 350 degrees and roast 45 minutes or longer, depending on desired doneness.Use your meat thermometer to determine time.