►HANGER STEAK: Mix together molasses, Worcestershire sauce, olive oil, garlic powder, celery seed, and ground mustard in a large bowl.
►If using large pieces of steak, cut into 5-inch pieces. (If pieces are small, there is no need to cut.) Toss with molasses mixture, cover, and marinate overnight in refrigerator.
►FETA VINAIGRETTE: Combine lemon zest and juice, vinegar, garlic, basil, capers, feta, salt, pepper, and red pepper flakes in a food processor and process until combined. With motor running, slowly add olive oil and process until emulsified. Measure out ¼ cup and reserve remainder for another use (see headnote).
►When ready to cook, heat a cast-iron skillet over high heat until it smokes. Remove steak from marinade, add to pan, and reduce heat to medium-low. Cook for 5 to 7 minutes, then flip steak. Cook for an additional 3 to 4 minutes for medium-rare.
►Rest steak for about 10 minutes, then cut into ½-inch-thick slices. Spoon vinaigrette over steak and serve.