Preheat oven to 350℉. Place cast-iron skillet in oven while it preheats.
Pour coarse ground pepper on a plate and press one side of flat iron steaks in pepper. Season steaks on both sides with salt.
Remove cast iron skillet from oven, add avocado oil, and place pepper side steaks down in pan. Return pan to oven. Bake for 4 minutes. Flip steaks and continue to cook in oven for an additional 4 minutes or until thermometer inserted in thickest part reads between 120-125℉ for a medium-rare steak.
Remove pan from oven and let steaks rest on room temperature surface for 8 minutes before slicing against the grain.