Standing Rib Roast With Two Sauces

1 beef rib eye roast (5-pound), 2 or 3 ribs
2 teaspoons whole black peppercorns, cracked
4 teaspoons fresh thyme leaves, divided
1 cup sour cream
1/4 cup Kraft prepared horseradish
1 teaspoon sugar
1 tablespoon butter
1 sweet onion, very thinly sliced
1 package (8-ounce) sliced fresh mushrooms
1 tablespoon flour
1 cup fat-free reduced-sodium beef broth
1 tablespoon Grey Poupon Dijon mustard

Heat oven to 350 degrees F.

Place meat, fat-side up, in roasting pan. Press peppercorns and 2 teaspoons thyme onto surface of meat. Bake 2 to 2 1/4 hours or until 135 degrees F. (To check for doneness, insert probe of instant-read thermometer 2 to 2 1/2 inches into thickest part of meat.) Meanwhile, mix sour cream, horseradish and sugar in serving bowl. Refrigerate until ready to serve.

Remove meat from oven. Cover meat with foil; let stand 15 to 20 minutes or until medium-rare doneness (145 degrees F).

Meanwhile, melt butter in large skillet on medium heat. Add onions; cook and stir 8 to 10 minutes or until golden brown. Add mushrooms; cook and stir 6 minutes or until tender. Sprinkle with flour; cook and stir 1 minute. Stir in broth; simmer 5 minutes or until thickened, stirring constantly. Remove from heat; stir in mustard and remaining thyme.

Slice meat. Serve with sauces.