Beef stroganoff with matchstick potatoes

Name: 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; 450g/1lb floury potatoes, such as Maris Piper, peeled; sunflower oil, for deep-frying