In a small bowl, whisk together the lime juice, chili powder, cumin, garlic powder, sea salt, and black pepper to create the marinade.
Rub the marinade evenly over both sides of the flank steak and allow it to marinate at room temperature for 15 minutes.
Heat the olive oil in a heavy cast-iron skillet over medium-high heat until the pan is shimmering.
Place the steak in the hot skillet and sear for 4 to 5 minutes per side for medium-rare, or until it reaches your desired level of doneness.
Remove the steak from the skillet and transfer to a cutting board, letting it rest for 5 minutes before slicing thinly against the grain.
Briefly warm the corn tortillas in a dry pan or over a low gas flame until they are pliable and slightly charred.
Divide the sliced steak among the tortillas and top with shredded cabbage, diced red onion, fresh cilantro, and avocado slices.