In a small bowl, whisk together the lime juice, ground cumin, garlic 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 rest for at least 15 minutes.
Heat the olive oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Sear the steak for approximately 4-5 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
While the meat rests, mash the avocado in a small bowl and stir in the finely diced red onion and chopped cilantro.
Slice the rested steak against the grain into thin, bite-sized strips.
Warm the corn tortillas in a dry pan over medium heat until they are pliable and slightly charred.
Assemble the tacos by dividing the steak between the tortillas, topping each with the avocado salsa and thinly sliced radishes.