Cut the meat into thin strips, sprinkle with salt and pepper and toss in flour. Heat the oil and half the butter in a heavy
frying pan and use this to gently fry the onions until transparent. Remove and keep warm. Add the meat and fry, sealing all sides
until golden brown.
Remove with a slotted spoon and keep warm with onion. Add rest of butter and sliced mushrooms to the frying pan and cook
gently for a further 5 minutes. Add remaining flour and stir in until smooth. Blend in the tomato puree with the hot stock, add to the pan and stir thoroughly. Return the onions and meat to the pan, bring to the boil, cover and simmer gently for 2-3 minutes. Add the soured cream and stir in well, remove from heat. cool and serve.