Pat the flank steak dry and season both sides evenly with chili powder, cumin, garlic powder, sea salt, and black pepper.
Heat the avocado oil in a grill pan or cast-iron skillet over medium-high heat until shimmering.
Place the steak in the pan and sear for 4-5 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Transfer the steak to a cutting board and let it rest for at least 5 minutes to lock in the juices.
While the steak rests, combine the avocado, Greek yogurt, and lime juice in a small blender or food processor and pulse until completely smooth.
Warm the corn tortillas in a dry skillet for 30 seconds per side until pliable.
Thinly slice the steak against the grain and divide it between the tortillas.
Top each taco with shredded cabbage, a generous dollop of avocado crema, and fresh cilantro.