2 tsp lemon juice
350g/12oz button mushrooms, thinly sliced
1 large onion, very thinly sliced
small handful of parsley leaves, finely chopped
freshly ground black pepper
1陆 tbsp paprika (hot Hungarian, if you like a little subtle heat)
salt
3 tbsp sunflower oil
sunflower oil, for deep-frying
65g/2陆oz unsalted butter
300ml/10fl oz soured cream
675g/1陆 lb beef fillet, preferably cut from the tail end