PLACE meat in large casserole dish; sprinkle with pepper. Pour Italian dressing over meat. Refrigerate 1 hour to marinate.
HEAT oven to 325°F. Remove meat from marinade; discard marinade. Place meat, fat side up, in roasting pan sprayed with cooking spray.
BAKE 2 to 2-1/2 hours or until 130°F to 135°F for medium-rare doneness. Remove from oven. Cover with foil; let stand 15 to 20 min. before slicing. Meanwhile, mix 1 cup Miracle Whip and horseradish in small bowl. Mix remaining Miracle Whip, Sweet Onion Dressing and thyme in separate bowl.
SLICE meat. Serve with both sauces.
serving size = 90 g meat + 1-1/2 Tbsp. (22 mL) sauce