In a baking dish just large enough to hold the steak, whisk together the balsamic vinegar, soy sauce, mustard, garlic, salt, and pepper. On a cutting board, pat the hanger steak dry with paper towels. Place the hanger steak in the marinade, flip it over, and use your hands or a spoon to coat it well. Cover with plastic wrap and let rest at room temperature for one hour. (Tip: You may marinate up to all day. In that case, refrigerate and then let sit at room temperature for 30 minutes before cooking.)
If cooking on the stovetop: Heat the oil in a 12- to 14- skillet over high heat. Once the oil begins to shimmer, remove the steak from the marinade, letting the excess drip off. Place the steak in the pan and cook without moving until charred, about 5 minutes. Flip the steak over with tongs and continue cooking for 4 minutes more or until the internal temperature of the steak reaches 140˚F. (Helpful hints: This will be pink but not raw in the middle. Keep in mind that cooking times will vary slightly based on precise thickness of steak and whether other cuts are substituted. A quick read thermometer is the best way to ensure the steak is cooked to your liking.)
If grilling: Preheat the grill to high. When the grill is hot, place the steak on the grill and cook for 4 to 5 minutes on each side for medium rare. For a little extra flavor, you may brush the steak occasionally with the excess marinade. When the internal temperature reaches 140˚F (medium rare), remove the steak from the grill.
Let the steak rest for 10 minutes before slicing. Serve topped with romesco. (Although it’s delicious all on its own too!)