Pat the flank steak dry with paper towels and season both sides evenly with sea salt, black pepper, garlic powder, and cumin.
Heat the olive oil in a heavy cast-iron skillet over medium-high heat until the oil is shimmering.
Place the steak in the hot skillet and sear for 4-5 minutes per side for medium-rare, or until it reaches your desired level of doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes to lock in the juices.
While the steak rests, quickly warm the corn tortillas in the same skillet or over an open flame until pliable and slightly charred.
Thinly slice the steak against the grain into bite-sized strips.
Finely dice the red onion and chop the fresh cilantro.
Assemble the tacos by dividing the steak strips between the two tortillas, then top with avocado slices, red onion, and cilantro.
Serve immediately with a fresh lime wedge for squeezing over the top.