Pat the flank steak dry with paper towels and season both sides evenly with garlic powder, cumin, chili powder, sea salt, and black pepper.
In a small bowl, mash the avocado with the Greek yogurt and half of the lime juice until smooth and creamy.
Heat the olive oil in a cast-iron skillet 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 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 for 30 seconds per side until pliable.
Thinly slice the steak against the grain into bite-sized strips.
Toss the shredded cabbage with the remaining lime juice.
Assemble the tacos by layering the steak strips onto the tortillas, followed by the cabbage, a dollop of avocado crema, and fresh cilantro.