Pat the flank steak dry and season both sides evenly with sea salt, black pepper, and ground cumin.
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 to 5 minutes per side for medium-rare doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes to retain juices.
While the steak rests, mash the avocado with the Greek yogurt and lime juice in a small bowl until smooth and creamy.
Warm the corn tortillas in a dry pan over medium heat for 30 seconds per side until pliable.
Slice the rested steak thinly against the grain into bite-sized strips.
Assemble the tacos by layering the steak into the tortillas, topping with shredded cabbage, a dollop of avocado crema, and fresh cilantro.