In a small bowl, whisk together the lime juice, garlic powder, cumin, salt, and pepper to create a marinade.
Rub the marinade onto the flank steak and let it sit for at least 15 minutes to absorb the flavors.
While the steak marinates, mash the avocado with the Greek yogurt in a small bowl until the crema is velvety and smooth.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 4-5 minutes per side until it reaches your desired level of doneness, then let it rest for 5 minutes.
Thinly slice the steak against the grain to ensure every bite is tender.
Warm the corn tortillas in a dry pan for 30 seconds per side until pliable.
Divide the steak among the tortillas and top with avocado crema, sliced radishes, diced red onion, and fresh cilantro.