Heat a large nonstick skillet coated with cooking spray over medium-high heat. Sprinkle steaks with pepper and salt. Add steaks to pan; cook 3 minutes on each side or until desired degree of doneness. Remove from pan.
Add mushrooms, onion, and garlic to pan; sauté 5 minutes. Stir in broth; bring to a boil. Cover, reduce heat, and simmer 3 minutes.
Place sour cream in a medium bowl. Gradually add mushroom mixture to sour cream, stirring constantly with a whisk. Stir in minced parsley. Serve gravy with steak.
Gradually add the hot mushroom mixture to the cold sour cream to keep the two from separating.