In a small bowl, whisk together the chili powder, smoked paprika, sea salt, black pepper, and half of the lime juice to create a rub.
Pat the flank steak dry with a paper towel and rub the spice mixture evenly over both sides of the meat.
Heat the olive oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 4-5 minutes per side for medium-rare, or until it reaches your desired doneness.
Transfer the steak to a cutting board and let it rest for 5 minutes to allow the juices to redistribute.
While the steak rests, toss the shredded cabbage with the remaining lime juice and chopped cilantro in a small bowl.
Warm the corn tortillas in a dry pan or directly over a low gas flame for 30 seconds per side until pliable and slightly charred.
Thinly slice the steak against the grain into bite-sized strips.
Assemble the tacos by dividing the sliced steak between the tortillas, topping with the zesty cabbage slaw and sliced avocado.