Take the roast out put on cutting board, leaving the juice in the pan. Take out the mushrooms and onion and fry them on high heat for about 10 minutes in a pan lightly coated with olive oil. Slice the roast as thin as possible, then put all of the meat back into roasting pan with original juice. Time to build sandwich. Put meat on a kummelweck roll, then top with mushrooms, onions, and Miller's Horseradish. Use the juice as the au jus.