Whisk the balsamic vinegar, soy sauce, mustard, garlic powder, and cumin in a small bowl.
Place the steak in a Ziploc bag. Pour the marinade into the bag and rub it all over the steak. Seal the bag, removing as much air as possible. Place the bag on a plate in the fridge for at least 4 hours or overnight. If you're home while the steak is marinating, flip it occasionally.
Cook the steak:
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, for about 3 minutes.
Remove the steak from the bag and pat it dry with paper towels. Sprinkle it on both sides with salt and pepper.
Place the steak in the hot skillet. Cook it for 3 minutes without moving.
Flip the steak to the other side (it should be deeply browned and slightly charred on the first side) and cook for 3 more minutes, again without moving it.
Using oven mitts, remove the skillet from the heat. Tent it with foil to allow the steak to rest and finish cooking from the skillet's heat for 5 minutes.
Transfer the steak to a cutting board. Slice it thinly and serve.
Video
Notes
If you have a frozen top round steak, it's perfectly fine to marinate it while it's still frozen. I often do. Simply place the frozen steak in a resealable bag with the marinade and place it in the fridge overnight.
If your cast-iron skillet is well seasoned, there's no need to add oil to the pan before frying the steak. If you'd like to add fat to the skillet, it's best to use fats appropriate for high-heat cooking, such as avocado oil or ghee.
The best way to ensure the steak is done to your liking is using an instant-read thermometer. Rare is 120-125°F (a warm red center). Medium-rare is 130-135°F (mostly pink center with some red in the middle). However, the CDC recommends cooking meat to an internal temperature of 145°F.
You can keep the leftovers in the fridge, in a sealed container, for 3-4 days. Thinly sliced and served cold, they make the perfect topping for salads.