Pat the steak dry and season both sides with salt, pepper, cumin and smoked paprika.
Drizzle olive oil and lime juice over the steak and rub to coat; let rest 20–60 minutes if time allows.
Preheat a heavy skillet or grill over high heat until very hot.
Sear the steak 3–5 minutes per side for medium-rare, adjusting for thickness.
Transfer the steak to a cutting board and let rest 5–10 minutes to retain juices.
Warm corn tortillas in a dry skillet or directly over a low flame until pliable; keep wrapped in a towel.
Slice the rested steak thinly against the grain into strips.
Assemble tacos by placing sliced steak on tortillas, then top with salsa, cilantro and pickled red onion.
Finish with a squeeze of fresh lime and optional cotija or avocado slices.
Serve immediately while tortillas are warm and steak is juicy.