Keeping the heat high, add the mushrooms to the same pan and cook, stirring, for 5 minutes until deep golden, softened and all their liquid has evaporated (add a tiny drizzle more oil if the mushrooms are sticking). Add the garlic and paprika, and fry for 2 minutes. Pour in the sherry vinegar and bubble for 1 minute until it disappears, then add the beef stock and simmer for 3 minutes. Turn down the heat and stir in the soured cream and any resting juices from the steak. Simmer for 2 minutes then season with pepper. Stir through the parsley then spoon over the steak and serve with the wedges and some watercress.