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Pour the fat out of the pan you cooked your steaks in, but don’t clean it. Place the pan back on the heat with half the butter and the shallots and sizzle for 2 mins to soften. Stir in the flour and cook to a sandy paste, then splash in the vinegar and simmer for a moment. Pour in the wine and stir, scraping any sticky bits off the bottom of the pan. Bring to the boil and bubble for a minute, then whisk in the mustard and then the stock.
Boil the sauce down to about 200ml in total, then taste and season. Whisk in the remaining butter and the parsley, plus any resting juices from the steaks. Serve the steaks on warm plates and spoon the sauce over the top.