In a small bowl, whisk together the lime juice, ground cumin, garlic powder, sea salt, and black pepper.
Rub the marinade all over the flank steak and let it sit for at least 15 minutes at room temperature.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Place the steak in the hot skillet and sear for 4 to 5 minutes per side to achieve a deep brown crust and medium-rare internal temperature.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain.
While the steak rests, warm the corn tortillas in a dry pan over medium heat for 30 seconds per side until soft and pliable.
Finely dice the red onion, chop the cilantro, slice the radishes, and slice the avocado.
Divide the sliced steak between the two tortillas and top with the avocado, onion, cilantro, and radishes.