In a small bowl, whisk together the lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper to create a marinade.
Place the flank steak in a shallow dish, coat thoroughly with the marinade, and let it marinate for at least 20 minutes at room temperature.
Prepare the avocado crema by mashing the avocado with the Greek yogurt and a squeeze of lime until the texture is velvety and smooth.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 4 to 5 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
While the steak rests, quickly toast the corn tortillas in the same skillet until they are warm and slightly charred.
Thinly slice the steak against the grain and divide it between the tortillas.
Top each taco with a dollop of avocado crema, thinly sliced radishes, diced white onion, and fresh cilantro leaves.