In a mixing bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, black pepper, and red pepper flakes.

Place the flank steak in a shallow dish and pour the marinade over it. Cover and refrigerate for at least 1 hour, or overnight.

Preheat grill or grill pan over medium-high heat.

Remove the steak from the marinade, allowing excess liquid to drip off.

Place the steak on the grill and cook for about 4-5 minutes per side for medium-rare.

Check the internal temperature with a meat thermometer; it should read 135°F for medium-rare.

Remove the steak from the grill and let it rest for 5-10 minutes.

Slice the steak against the grain into thin strips.