Prepare the grill for two zone cooking, or direct/indirect using lump charcoal. Target 500 degrees Fahrenheit over the direct heat. Place cast iron pan over the indirect side while the grill warms up.
Remove the flank steak from the marinade and discard marinade. Combine the salt, pepper and garlic in a small bowl and season the steak.
Start by placing the flank steak over direct heat. As soon as you place the flank steak over direct heat, drop all the peppers and onions into the cast iron pan and stir. It will sear and smoke.
While the peppers and onions cook, sear the steak for four minutes or until you see good grill marks over the direct heat. Flip the flank steak and sear over direct heat for an additional 4 minutes.
Move the flank steak to the indirect side. At the same time, using high heat gloves, move the cast iron pan over the direct heat and continue to stir the peppers and onions.
Cook the flank steak over indirect heat until the internal temperature of the flank steak at the thickest part reads 125 degrees F, about 4 - 6 more minutes. Remove the flank and let it rest for 10 minutes.
While flank is cooking and resting, continue stirring the peppers and onions until they are soft. This should take a total time of 16 - 18 minutes. Likely the flank will be resting while the veggies finish up.
Remove the cast iron from the grill when the peppers and onions are soft. Slice the flank steak against the grains in thin slices and serve with your favorite toppings.