Make the marinade by combining the soy sauce, rice vinegar, sesame oil, brown sugar, ginger root, minced garlic, and crushed red pepper flakes in a zip-lock bag. Add the flank steak, squish it around until evenly coated, and refrigerate for 1-24 hours.
20 minutes before cooking, remove it from the fridge, remove the steak from the marinade & place it on a few paper towels to dry off the liquid.
Cook the flank steak by heating a grill pan coated with olive oil cooking spray over medium-high heat.
Once hot, add the flank steak and cook for 4-5 minutes per side until reaching the desired degree of doneness. Remove from the grill to a cutting board.
Let the steak rest for at least 5 minutes before slicing thinly against the grain. Serve and enjoy.