onion, finely chopped
Discard bone and fat from meat and cut into thin, bite-size strips.
Heat 1/2 tablespoon of the butter with the oil in frying pan over medium-high heat.
Add mushrooms and onion and cook, stirring until golden.
Add remaining 1/2 tablespoon of butter to pan.
Add beef strips and cook over high heat, turning meat and shaking pan, just until browned.
Remove pan from heat, stir in reserved mushroom-onion mixture, salt, worcestershire, paprika, pepper and sour cream.
Return pan to low heat and cook, stirring constantly, just until sauce is heated through--do not boil.
Serve over noodles or rice.