Remove any fat from the beef and discard. Cut the beef into thin strips.
Heat half the oil in a frying pan, add the beef strips and brown for 1-2 minutes over high heat (if necessary, brown meat in batches to avoid having too much in the pan at one time). Transfer the browned beef to a warm plate and set aside.
Reduce heat, add the remaining oil to the pan and sauté the onion until soft. Add the stock and return the meat to the pan. Stir in the Worcestershire sauce, tomato paste, paprika and pepper, then cover and simmer gently for up to 1 hour or until tender.
Add the mushrooms to the pan and cook for 15 minutes.
Blend together the yogurt and flour in a small bowl and slowly add to the simmering pan, stirring well. Heat gently without boiling until thickened.
Garnish with the yogurt and chopped parsley and serve with hot buttered noodles or mashed potatoes.
For a more economical family dish use lean mince instead of steak.
Replace half of the beef stock with the same amount of red wine.