In a large frying pan over medium-high heat, heat the oil and brown the steak strips and onion. When done remove the steak and onion from the pan.
To the same pan add butter and melt over low heat. Whisk in the flour and stir with a wooden spoon until smooth. Whisk in the beef broth and cook until the mixture thickens slightly, about 2 minutes. Season with salt and pepper.
Add mushrooms and return the beef and onions to the pan. Simmer for 30 minutes, until tender. Worcestershire sauce and the sour cream are added at the very end.
Do not allow the topping to boil. Keep hot in a chafing dish over very low heat. Serve over baked potatoes.