Skillet Beef Steak with Avocado Sauce

POUR 1/3 cup dressing over steaks in shallow dish; turn to coat both sides of each steak. Refrigerate 30 min. to marinate. Meanwhile, cook tomatillos in 2 cups simmering water in saucepan on low heat 15 min. or until tomatillos are softened and skins are tender. (Do not let tomatillos boil.)

DRAIN tomatillos, reserving 1/2 cup cooking liquid; place tomatillos and reserved liquid in blender. Add avocados, 1/4 cup cilantro, 1/2 cup onions and garlic; blend until smooth.

HEAT remaining dressing in large skillet on medium-high heat. Add onions; cook on medium heat 5 min. Remove from skillet; cover to keep warm. Remove steaks from marinade; discard marinade. Add steaks, in batches, to skillet; cook 2 min. on each side or until medium doneness (160ºF). Return steaks and onions to skillet. Top with avocado sauce. Cook on low heat 1 to 2 min. or until sauce is heated through. Serve topped with sour cream and remaining cilantro.