Pat the flank steak dry with a paper towel to ensure a proper sear.
In a small bowl, combine the sea salt, black pepper, chili powder, and cumin. Rub the spice blend evenly over both sides of the steak.
Heat the olive oil in a cast-iron skillet or heavy pan over medium-high heat until shimmering.
Place the steak in the hot skillet and sear for 4-5 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes to keep the juices intact.
While the steak rests, finely dice the red onion, chop the cilantro, and slice the avocado into thin wedges.
Briefly warm the flour tortillas in a dry pan over medium heat for 30 seconds per side until soft and pliable.
Thinly slice the rested steak against the grain into bite-sized strips.
Assemble the tacos by dividing the steak between the tortillas, then topping with avocado, red onion, and cilantro.
Finish with a fresh squeeze of lime juice over the top before serving.