In a small bowl whisk together soy sauce, vinegar, water, brown sugar, garlic, ginger, and red pepper flakes if added.
Place the steaks in a container or large ziplock bag and pour ½ of the marinade over the steaks. Reserve the rest.
Marinade the steak for 4 hours or overnight for the best flavor. You will want to give the steak a toss a couple of times during the marinating process.
Place the remaining sauce in a small saucepan, mix the cornstarch with 1 teaspoon of water to make a slurry then add it to the sauce and whisk.
Remove the steaks from the refrigerator and let them come to room temperature for 30 minutes.
Simmer the sauce on medium-high heat for about 12-15 minutes or until thickened. Remove from heat and let cool.
Heat a grill on medium-high heat and place the steak over the heat. Grill for 2-3 minutes then flip and grill another 2-3 minutes over direct heat. Then move the steaks over to indirect heat and cook. Baste the steaks with the thickened teriyaki sauce. I recommend using a meat thermometer to know when the steaks have reached your desired doneness.
Remove from the grill and let rest 5 minutes before slicing.
Notes
Let the steaks come to room temperature before placing them on the grill.Cooking time can vary if your steaks are super thin or really thick. That's why it's so important to use a digital meat thermometer.You can use 1/4 teaspoon ground ginger in place of the fresh.