- 675g/1½ lb beef fillet, preferably cut from the tail end
- 65g/2½oz unsalted butter
- 1½ tbsp paprika (hot Hungarian, if you like a little subtle heat)
- 1 large onion, very thinly sliced
- 350g/12oz button mushrooms, thinly sliced
- 3 tbsp sunflower oil
- 300ml/10fl oz soured cream
- 2 tsp lemon juice
- small handful of parsley leaves, finely chopped
- salt
- freshly ground black pepper
- 450g/1lb floury potatoes, such as Maris Piper, peeled
- sunflower oil, for deep-frying
- Cut the steak into slices 1cm/0.5in thick, then cut each slice across the grain into strips 1cm/0.5in wide.
- For the matchstick potatoes, cut the potatoes by hand into short sticks 3mm/0.2in thick or use a mandoline. Set aside in a bowl of cold water.
- Heat the sunflower oil for deep-frying, either in a large pan or in a deep-fat fryer, to 190C/375F.
- Meanwhile, melt the butter in a large frying pan, add the paprika and onion and cook slowly until the onion is soft and sweet, but not browned.
- Add the mushrooms and fry gently for three minutes. Transfer the mixture to a plate and keep warm.
- Drain the matchstick potatoes and dry thoroughly in a clean tea towel or a salad spinner.
- Plunge the potatoes into the hot oil and fry for three minutes, until crisp and golden. Drain briefly on kitchen paper and keep hot in a low oven.
- Using the same pan you cooked the onion mixture in, heat one and a half tablespoons of sunflower oil until very hot.
- Add half the fillet steak and fry quickly, seasoning and turning it as you do so, for just over one minute. Transfer to a plate and repeat with the rest of the oil and steak.
- Return the onion mixture to the pan and pour in the soured cream. Bring to the boil and simmer for a minute or so, until thickened.
- Return the steak to the pan and heat very gently for one minute – the beef should not be cooked any further.
- Stir in the lemon juice and parsley and serve with the matchstick potatoes.