Season the flank steak evenly on both sides with ground cumin, garlic powder, sea salt, and black pepper.
Heat olive oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Sear the steak for approximately 4 to 5 minutes per side for medium-rare, or until it reaches your preferred doneness.
Transfer the steak to a cutting board and allow it to rest for 5 minutes to lock in the juices.
While the meat rests, mash the avocado with the Greek yogurt and half of the lime juice in a small bowl until the texture is velvety.
Warm the corn tortillas in a separate dry pan over medium heat for about 30 seconds per side until pliable.
Slice the rested steak thinly against the grain to ensure every bite is tender.
Assemble the tacos by dividing the steak among the tortillas, then topping with the avocado crema, sliced radishes, fresh cilantro, and the remaining lime juice.