1 - Heat half the oil in a large skillet over medium-high heat. Season 2 steaks with 3/4 teaspoon salt and pepper. Cook until deep golden brown and an instant-read thermometer inserted into the center reaches 130F, about 4 minutes per side (for medium-rare). Transfer to a platter, and tent with foil. Repeat with remaining oil and steaks. Let steaks rest in a 250F oven.
2 - Reduce heat to medium, and add wine to skillet. Cook until almost completely reduced, about 5 minutes. Add beef stock, and cook until reduced to 1/4 cup, about 2 1/2 minutes. Reduce heat to low, and whisk in butter (1 piece at a time) and mustard. Stir in any juices from the platter, and season with salt and pepper. Spoon sauce over steaks.
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