In a medium bowl, stir together balsamic vinegar, Worcestershire sauce, brown sugar, honey, Dijon mustard, garlic powder, and minced onion. If it’s runny, stir in another tablespoon of brown sugar.
Put the steak in a shallow container or zippered freezer bag and pour the marinade over it so that it covers the steak. Place the container or bag in the refrigerator overnight. If you don’t have that much time to spare, allow the steak to marinate in the fridge for at least one hour.
Before cooking, remove the steak from the bag and allow to sit on a cutting board at room temperature for 30 minutes.
Meanwhile, preheat a cast-iron skillet over medium-high heat with oil.
Add the steak to the pan and sear for 3-4 minutes on each side. The internal temperature should reach 135 degrees for medium-rare.
Place the steak on a cutting board and allow to rest for 5 minutes before slicing into thin strips and enjoying.