Cutting around the core like you would an apple, quarter the peppers. Broil or grill until blackened in spots and softened, 7-10 minutes, depending on your broiler/grill situation. When cool enough to handle, slice into taco-sized strips, season lightly with salt, and set aside.
Set a large cast-iron skillet over medium-high heat and allow to heat while you prepare the steak.
Pat steak dry with paper towels, then season generously with salt.
Swirl oil into pan, and add steak. Allow to cook, undisturbed, until deeply browned, 2-4 minutes depending on the thickness of your steak. Flip to brown briefly, another 2-3 minutes. Allow steak to rest for at least 10 minutes before carving into it.
While steak is resting, char tortillas over stovetop burners, 20-30 seconds each side. Pick your cilantro leaves, chop your onion, gather your roasted peppers.
To assemble: lay strips of steak and peppers across the middle of each tortilla. Shower with cilantro and onion.