Combine the salt, pepper, cumin, and cayenne in a small bowl. Rub the steak with the seasoning on both sides. Heat the oil in a 10-inch heavy skillet (preferably cast iron) over medium-high heat until very hot but not smoking. Put the steak in the hot pan and sear for 3 minutes. Turn the steak and cook for another 2 minutes. (it will be very rare but will cook more as it rests). Take the skillet off the heat, transfer the steak to a cutting board, and cover loosely with foil.
Return the skillet to medium heat. Add the pepper and onion slices and sauté, stirring frequently, until soft, about 8 minutes. Add the jalapeño and garlic and cook until soft and fragrant, about 1 minute. Slide the skillet off the heat and stir in the lime juice, scraping up any browned bits from the pan. Cover to keep warm.
Trim the fat from the side of the steak and cut the steak into thin (1/8- to 1/4-inch) slices. Add the slices to the skillet along with any accumulated juices. Toss until well blended. Stir in the chopped cilantro and season with salt and pepper. Serve immediately with warm flour tortillas, sliced avocado, sour cream, chopped tomatoes, and cilantro sprigs.