Pour 1/4 cup dressing over steak and seal in a Ziploc bag. Refrigerate at least 30 minutes or overnight to marinate.
Combine sour cream, 2 tbsp of the remaining dressing, Parmesan cheese, salt and pepper. Refrigerate until serving time.
Heat remaining 2 tbsp dressing and garlic in large nonstick skillet on medium heat. Add onions; cook until golden brown, stirring frequently. Remove onions; set aside.
Drain steak; discard marinade. Cook steak in same skillet on medium heat 3-4 on each side for medium doneness. Place on cutting board; slice crosswise into thin strips. Serve steak topped with sauce and onions.