Place the shallots, butter, and lots of black pepper in a small saucepan over medium-low heat. Sweat the shallots until almost tender without browning. Add the wine, bring to a boil, and reduce totally. Set aside.
Heat a nonstick frying pan over high heat. Brush one side of the flank steak with a thick coating of mustard and placemustard side down in the pan. Cook for 4 minutes. Brush the other side with mustard and cook that side for 4 minutes. Remove the pan from the heat, top the steak with the shallots, cover, and set aside for 5 minutes.