700g beef fillet, preferably cut from the tail end
65g unsalted butter
1½ tbsp paprika (hot Hungarian, if you like a little subtle heat)
1 large onion, very thinly sliced
350g button mushrooms, thinly sliced
3 tbsp oil
300ml soured cream
juice of half lemon
small handful of parsley leaves, finely chopped
freshly ground black pepper
500g floury potatoes, such as Maris Piper
Cut the steak into slices 1cm/0.5in thick, then cut each slice across the grain into strips 1cm wide
For the matchstick potatoes, cut the potatoes by hand into short sticks 3mm thick or use a mandolin. Set aside in a bowl of cold water.
Heat a deep-fat fryer to 190C
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
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 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