Season the flank steak evenly on both sides with sea salt, black pepper, ground cumin, and garlic powder.
Heat the olive oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 4 to 5 minutes per side for medium-rare, or until your desired doneness is reached.
Transfer the steak to a cutting board and let it rest for 5 minutes to lock in the juices before slicing thinly against the grain.
While the steak rests, mash the avocado in a small bowl with the Greek yogurt, lime juice, and a pinch of salt until the texture is velvety.
Warm the corn tortillas in a separate dry pan over medium heat for 30 seconds per side until soft and pliable.
Divide the sliced steak between the tortillas, then top each with a dollop of avocado crema, diced red onion, sliced radishes, and fresh cilantro.