MIX sour cream, 2 Tbsp. dressing, Parmesan and pepper. Refrigerate until ready to serve.

HEAT remaining dressing in large nonstick skillet on medium heat. Add onions; cook 7 to 8 min. or until golden brown, stirring frequently. Remove onions from skillet; set aside.

COOK steak in same skillet on medium heat 3 to 4 min. on each side or until done. Place on cutting board; cut across the grain into thin strips. Serve topped with sauce and onions.