In a hot saute pan melt butter and saute the mushrooms first then add the onions. The mushrooms take a little longer. In a separate pan brown strips of sirloin that have been dredged in flour. Set aside. When the onions and mushrooms are done, deglaze both of the pans with a little white or red wine and combine all ingredients into one pan. Add 2 cups beef stock, white wine or water and 1/2 of the pint of sour cream. Simmer for 30 to 45 minutes. During the cooking the sour cream will separate and some of the butterfat will become visible. This is normal, just stir the pot. Just before serving add the balance of the sour cream, stir to combine. Serve over noodles or rice pilaf and garnish with parsley.