Heat 2 teaspoons of oil in a large non-stick frying pan over medium heat. Add onion. Cook for 3 minutes. Increase heat to high. Add mushrooms. Cook for 4 minutes, or until tender. Set aside.
Meanwhile, place flour, and salt and pepper into a shallow dish. Lightly coat beef strips in flour. Add 2 teaspoons of oil to frying pan. Add half the beef. Cook over high heat until meat is browned, shaking pan to turn meat. Remove. Repeat with remaining beef and oil.
Return beef, onion and mushrooms to frying pan. Add stock and Worcestershire sauce. Bring to the boil. Reduce heat to medium-low. Simmer for 8 minutes, or until sauce has thickened slightly. Season with salt and pepper. Remove from heat. Stir through sour cream.
Spoon pasta onto plates. Spoon over stroganoff. Serve with steamed green beans.