Whisk together 2 tablespoons of sesame oil with the garlic, salt, onion powder, Cajun seasoning, hot pepper sauce, soy sauce, teriyaki sauce, and brown sugar in a bowl, then pour into a resealable plastic bag. Add the flat iron steak, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 12 hours.
Remove the flat iron steak from the marinade, and shake off excess. Discard the remaining marinade. Heat the remaining 3 tablespoons of sesame oil in a cast iron skillet over medium high heat until it begins to smoke. Sear the steak in the hot oil until cooked to your desired degree of doneness, about 3 minutes per side for medium. Remove the steak from the skillet and set aside to rest for 5 minutes before slicing thinly. While the steak is resting, cook the bell pepper in the hot skillet until it begins to soften, about 1 minute. Stir in the green onions, and cook for 30 seconds more. Serve the pepper mixture over the sliced steak.
