Whisk together 1/4 cup oil, the vinegar, garlic, mustard, + Worcestershire sauce ina large glass dish. Place steak in dish; turn to coat with marinade. Let steak marinate, turning once, 20 minutes.

Heat 2 tbsp oil in a large skillet over medium-high heat. Add shallots; cook, stirring often, until just golden, 2 to 3 minutes. Reduce heat to medium-low. Season with salt. Cook, adding 1/4 cup water in batches as needed to keep shallots from sticking, until tender and caramelized, 15 to 18 minutes Transfer shallots to a plate.

Wipe out skillet. Heat 2 tbsp oil over medium-heat heat. Remove stek from marinade; pat dry. Season with salt + pepper. Cook steak, turning once, until an instant-read thermometer registers 140°F for med-rare, 10 - 12 minutes per side. Tent with foil; let stand at room temperature 10 minutes. Season with pepper.

Meanwhile, wipe out skiooet; reheat shallots over medium heat. Thinlyslice steak, Serve with shallots and mustard.