COOK and stir onion in 1-1/2 Tbsp. of the oil in large skillet on medium-high heat for 3 minutes. Add mushrooms; cook and stir just until tender. Remove from skillet; cover to keep warm.
HEAT remaining 1-1/2 Tbsp. oil in same skillet. Add meat, in batches, to skillet; cook and stir until meat is cooked through. Return all meat to skillet. Stir in onion mixture and steak sauce; cook until heated through, stirring occasionally. Remove from heat.
STIR in sour cream. Serve over noodles. Garnish with chopped fresh parsley, if desired.
