Place flank steak in a shallow pan or plate.
If grilling, in a medium bowl, whisk together soy sauce, garlic, seasoned salt, pepper, and brown sugar. Pour the mixture over the steak. Flip the steak over a few times to make sure the marinade has fully covered both sides of the steak. Let the steak sit in the marinade for 10 to 15 minutes, flipping a few times.
If pan frying, use a dry rub: to make the dry rub, mix together 1/4 teaspoon seasoned salt, 1/4 teaspoon black pepper, 1/2 teaspoon garlic powder, 1/2 teaspoon onion powder, and 1 teaspoon brown sugar. Rub this spice mixture in both sides of the stake with your hand, working the spices into the meat. Immediately cook.
Grilling: After the meat is done marinating, place it on a hot grill or under broiler on high. Sear the steak on high for 5 minutes on each side. Then turn the grill down to medium and continue to cook for 10 minutes on each side or until desired doneness.
Cast Iron Pan: heat a cast iron skillet or other heavy skillet on medium. Add about a tbsp of vegetable oil. Brown flank steak 2-3 minutes per side. Then place the skillet in a 325° oven for 10-15 minutes or until desired doneness.
For best results, the internal temperature should be between 140° to 150°.
Once the meat is cooked, let it rest for about 10 minutes or all the juices will run out. Cover it with foil to keep it warm. This particular cut of beef will be too tough if you don’t slice it correctly. You want to cut it into thin slices at a 45-degree angle across the grain.