When oil is heated, add thinly sliced steak and brown. Next add diced onion and cook for a couple minutes. Finally, add sliced mushrooms and saute with steak and onion for another couple minutes. Season with salt and pepper to taste.
Mix sour cream with flour until combined (this will be your thickener). Mix sour cream mix into steak, onion, mushrooms. Mix well. Pour in 2 cups milk and cook until mixture thickens. May add more milk if sauce is too thick. Check seasoning and add more salt and pepper to taste.
Serve over noodles or rice.