In a small bowl, whisk together 1 tbsp lime juice, olive oil, cumin, garlic powder, salt, and pepper.
Place the flank steak in a shallow dish, coat thoroughly with the marinade, and let sit for at least 15 minutes.
Heat a cast-iron skillet over medium-high heat and sear the steak for 4-5 minutes per side until medium-rare.
Remove the steak from the heat and let it rest on a cutting board for 5 minutes before slicing thinly against the grain.
While the steak rests, blend the avocado, Greek yogurt, remaining lime juice, and cilantro in a small food processor until smooth.
Warm the corn tortillas in a dry pan over medium heat until soft and slightly charred on the edges.
Assemble the tacos by layering the sliced steak into the tortillas, then topping with avocado crema, radishes, and white onion.