In a shallow baking dish, whisk together the olive oil, Worcestershire sauce, vinegar, mustard, rosemary, and garlic.
Add the hanger steaks to the marinade, turn to coat, cover, and refrigerate at least 2 hours and up to 12 hours, flipping the steaks once for an even marinade.
Thirty minutes before you are ready to cook, remove the steak from the refrigerator. Remove the steak from the marinade and pat dry. Sprinkle all over with the salt and pepper.
Heat a grill to medium-high heat (about 400˚F to 450˚F) for 5 to 10 minutes until hot. Scrub the grill grate with a grill brush. With a pair of tongs, dip a paper towel or clean rag in some vegetable oil (do not soak it) and wipe it over the grates.
Place the steaks directly over the flame and cook, covered, until a thermometer inserted in the thickest part reaches 135˚F, 5 to 7 minutes per side for medium-rare.
Transfer to a cutting board, tent with foil, and let rest for 5 minutes before slicing against the grain and sprinkling with flaky salt.
