Rub the flank steak evenly with minced garlic, cumin, smoked paprika, sea salt, and black pepper.
Heat the avocado oil in a cast-iron skillet over high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 4 minutes per side to achieve a deep smoky char and medium-rare finish.
Transfer the steak to a cutting board and let it rest for 5 minutes to retain its juices before slicing against the grain into small cubes.
Briefly toast the corn tortillas in a separate dry pan over medium heat until they are warm and pliable.
Divide the cubed steak between the tortillas and top with diced white onion, chopped cilantro, and thinly sliced radishes.
Serve the tacos immediately with fresh lime wedges for squeezing over the top.