In a small bowl, whisk together the olive oil, lime juice, cumin, garlic powder, sea salt, and black pepper to create the marinade.
Rub the marinade thoroughly over the flank steak and allow it to rest for at least 15 minutes at room temperature.
Heat a cast-iron skillet or grill pan over medium-high heat and sear the steak for 4-5 minutes per side until it reaches your desired doneness.
Transfer the steak to a clean cutting board and let it rest for 5 minutes to lock in the juices.
While the meat rests, mash the avocado with the Greek yogurt and a pinch of salt in a small bowl until smooth and velvety.
Warm the corn tortillas in a dry skillet for about 30 seconds per side until they are soft and pliable.
Thinly slice the steak against the grain into bite-sized strips.
Assemble the tacos by layering the steak into the tortillas, followed by a dollop of avocado crema, sliced radishes, diced red onion, and fresh cilantro.