Melt half the butter in a large deep frying pan and fry the onions until pale golden. Add the mushrooms and green pepper to the pan and cook for 5 minutes. Remove the onions, mushrooms and green pepper from the pan.
Melt the remaining butter and heat, then fry the steak strips for about 4 minutes, turning so they are cooked evenly.
Return the onions, mushrooms and peppers to the pan and season well, then stir in the soured cream and blend well. Heat until piping hot, but do not allow to boil. Garnish with chopped parsley.