1. 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, 1 to 2 minutes. Remove from heat, transfer steak to a plate, and set aside.

2. 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 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 one side of the steak, increase the heat to medium, and cook, turning the steak after 3 or 4 minutes for medium-rare. Season with salt and pepper to taste. Transfer the steak to a serving dish with the pan juices spooned over the steak. Garnish with a sprig of parsley.