Heat a very large heavy skillet over medium-high heat. Add the oil and swirl it around to coat the pan.
Saute the steaks about 4 minutes each side (this is for rare—cook longer if more doneness is dersired). Remove from pan and keep the steaks warm.
Place the mushrooms in the same pan and sear for about 3 minutes or until they start to caramelize. Add the onion and saute for 3 minutes. Then stir in the garlic and tomato paste and cook for 1 minute. Sprinkle with flour and cook for 1 minute.
Deglaze the pan with the sherry, scraping up all the browned bits (fond) at the bottom of the pan. Add the stock, salt, pepper and Worcestershire sauce and simmer until thickened; about 3 minutes.
Remove the pan from the heat and stir in the sour cream. Add more salt and/or pepper to taste. Garnish with the parsley.
NOTES: Use a good-quality stock. I use Better Than Bouillon—about 1 tbs to the 1 1/2 cups water. Low-sodium stocks are preferred