Heat a large, heavy skillet over medium-high heat until it begins to smoke. Add steak and cook until nicely browned, 1 to 2 minutes. Turn and cook until browned on the other side, 1 to 2 minutes. Remove from heat and transfer steak to a plate and set aside.
When the skillet has cooled slightly, return it to the stove over medium-low heat. Add the butter; when it melts, add the shallots and saute for 1 minute. Using a spoon, stir in the sherry vinegar until the sauce thickens. Return the steaks to the skillet along with any accumulated juices. Spread the mustard over 1 side of the steak and increase the heat to medium; cook, turning the steak after 3 or 4 minutes for medium rare. Season with salt and freshly ground black pepper, to taste. Transfer the steak to a serving dish with the pan juices spooned over the steak. Garnish with a sprig of parsley.
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