Pat the flank steak dry with paper towels and season both sides evenly with the sea salt, black pepper, garlic powder, and ground cumin.
Heat the extra virgin olive oil in a heavy cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the skillet and sear for approximately 4 to 5 minutes per side for a medium-rare finish, ensuring a deep, flavorful crust develops.
Transfer the steak to a clean cutting board and allow it to rest for at least 5 minutes to retain its juices.
While the meat rests, combine the avocado, non-fat Greek yogurt, and lime juice in a small bowl, mashing until the mixture is velvety and smooth.
Warm the corn tortillas in a dry skillet over medium heat for 30 seconds per side until they are soft and pliable.
Slice the rested steak against the grain into thin strips and divide them among the warm tortillas.
Finish each taco with a dollop of avocado crema, a sprinkle of diced red onion, and fresh cilantro.