1. Cut the beef into bite size strips (about 1 inch wide by 2 1/2 inch long strips).
2. Slice the mushrooms and chop the onion.
3. Heat 1 tablespoon oil in a large skillet over medium-high heat. Brown the beef strips on each side.
4. In a separate skillet, cook mushrooms and onions in 2 tablespoons oil over medium heat until soft.
5. Transfer mushrooms and onions to the skillet with beef. Season with salt and pepper to taste.
6. Stir in sour cream, reduce heat to low and continue cooking until sour cream is hot but not boiling (if sour cream comes to a boil, it will curdle).
7. Serve stroganoff over egg noodles or rice.