In a small bowl, whisk together half of the lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper to create a zesty marinade.
Place the flank steak in a shallow dish, coat thoroughly with the marinade, and let it rest for 20 minutes to tenderize.
While the steak marinates, mash the avocado with Greek yogurt and the remaining lime juice until the crema is velvety and smooth.
Heat the olive oil in a cast-iron skillet over medium-high heat and sear the steak for 4 minutes per side for a perfect medium-rare.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing it thinly against the grain.
Warm the corn tortillas in a dry pan for 30 seconds per side until they are soft and pliable.
Assemble the tacos by filling the tortillas with steak and topping them with avocado crema, sliced radishes, red onion, and cilantro.