Heat a medium skillet on medium/ high heat. While the pan is heating mix up teriyaki sauce by grating garlic and ginger, and then mixing with soy sauce, sesame oil, brown sugar, and corn starch. Stir until it is well incorporated.
Salt and Pepper Flat Iron Steaks and lay in hot skillet. Brown both sides, but do not fully cook. You will know when to turn steaks because they will come free of the pan without sticking.
Remove browned steaks from the pan and cut into 1" pieces. Return to the pan and add teriyaki sauce. Stir continuously. As the teriyaki sauce heats up, it will cost the steak and get the sticky glaze that makes it so good.
It should only take 2-3 minutes. Immediate remove steak from the pan to prevent over cooking.