Season steaks with salt and pepper. Saute in oil over medium-high heat about 4 minutes on each side for rare, or to desired doneness. Remove and keep warm.
Add mushrooms to pan and saute 3 minutes. Stir in onions and cook 3 minutes more. Stir in garlic and tomato paste and cook 1 minute.
Sprinkle with 3 tablespoons flour and cook 1 minute. Deglaze pan with small amount of beef broth (or 1/2 cup dry sherry), scraping up browned bits from the pan.
Add the rest of the broth and Worcestershire, and simmer until thickened, 2-3 minutes.
Take off heat and stir in sour cream.
Pour sauce over steaks, then garnish with parsley.