POUR 1/4 cup of the dressing over steak in resealable plastic bag. Seal bag; turn to evenly coat steak with dressing. Refrigerate 30 min. to marinate. Meanwhile, mix sour cream, 2 Tbsp. of the remaining dressing, the Parmesan cheese and pepper; cover. Refrigerate until ready to serve.

HEAT remaining 2 Tbsp. 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.

REMOVE steak from marinade; discard bag and marinade. Add steak to skillet; cook on medium heat 3 to 4 min. on each side or until medium doneness (160ºF). Remove steak from skillet; cut into thin strips. Serve topped with the sour cream sauce and onions.