Beef stroganoff with matchstick potatoes
Ingredients 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