2tablespoonssoy sauce (or use a gluten-free alternative and add salt as needed)
1.5lbs.top round steak,1 - 1.25 inches thick
1teaspoonDiamond Crystal kosher salt(not fine salt)
In a small bowl, whisk together the balsamic vinegar, soy sauce, mustard, garlic powder and cumin. Place the steak in a ziploc bag. Pour the marinade into the bag and rub all over the steak. Seal the bag, removing as much air as possible. Place the bag in the fridge for 4 hours or overnight. If you're home while the steak is marinating, flip it once in awhile.
Remove the steak from the fridge 1-2 hours before cooking, to allow it to reach room temperature.
Heat a well-seasoned cast iron skillet over medium-high heat until very hot, about 3 minutes.
Remove the steak from the bag and pat dry with paper towels. Sprinkle with salt and pepper. Place the steak in the hot skillet. Cook it 3 minutes, without moving. Flip to the other side (it should be deeply browned and slightly charred on the first side) and cook 3 more minutes, again without moving it.
Using oven mitts, remove the skillet from the heat. Tent it with foil, and allow the steak to rest - and finish cooking from the skillet's heat - for 5 minutes. The best way to ensure that the steak is done to your liking is using an instant-read thermometer. Rare is 120-130 degrees F (a warm red center). Medium-rare is 130-135 degrees F (mostly pink center with some red in the middle).
Transfer the London Broil to a cutting board. Slice it very thinly, and serve.
Consuming undercooked meat may increase your risk of foodborne illness. I always cook lamb and beef steaks, chops and roasts to medium-rare. But you should make your own decision.