Roast Prime Rib with Horseradish Sauce

1 (8 pound) prime rib roast
1 tablespoon dry mustard
1 tablespoon ground black pepper
1 tablespoon soy sauce
1 medium onion, sliced
Horseradish Sauce

Heat oven to 450 degrees F.

Rub the seasonings on the roast and place on a rack in roasting pan. Place the onions on top of the roast. Place in oven for 25 minutes.

Reduce heat to 325 degrees F, and roast for one hour longer or until internal meat temperature reaches 115 degrees for rare. Cook an additional 20 minutes for medium.

Let stand 15 minutes before carving.

Horseradish Sauce:
1/4 cup prepared horseradish
1/2 cup mayonnaise
1/4 cup sour cream

Combine all ingredients and chill. Serve with prime rib.