In a small bowl, whisk together the lime juice, garlic powder, ground cumin, chili powder, sea salt, and black pepper to create the marinade.
Place the flank steak in a shallow dish and rub the marinade evenly over both sides; let it marinate at room temperature for 15 minutes.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the flank steak for 4-5 minutes per side for medium-rare, or until it reaches your desired level of doneness.
Transfer the steak to a cutting board and allow it to rest for 5 minutes to lock in the juices.
While the steak rests, mash the avocado with the Greek yogurt and a squeeze of extra lime juice in a small bowl until smooth and velvety.
Warm the corn tortillas in a dry pan over medium heat for 30 seconds per side until pliable.
Slice the steak thinly against the grain and distribute evenly between the tortillas.
Top each taco with a dollop of the avocado crema and a garnish of fresh cilantro before serving.