Whisk together the lime juice, olive oil, minced garlic, cumin, chili powder, salt, and pepper in a small bowl.
Place the flank steak in a shallow dish, coat thoroughly with the marinade, and let it marinate for at least 15 minutes at room temperature.
Prepare the crema by mashing the avocado with the Greek yogurt and a pinch of salt until the texture is velvety and smooth.
Heat a cast-iron skillet over high heat and sear the steak for 4-5 minutes per side until it reaches a juicy medium-rare internal temperature.
Transfer the steak to a cutting board and allow it to rest for 5 minutes before slicing thinly against the grain.
Briefly toast the corn tortillas in the hot skillet for 30 seconds per side until they are warm and pliable.
Assemble the tacos by dividing the steak between tortillas and topping with the avocado crema, thinly sliced radishes, and fresh cilantro.