Pat the flank steak dry with paper towels and season both sides evenly with sea salt, black pepper, and garlic powder.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 4-5 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
While the steak rests, finely dice the white onion, chop the fresh cilantro, and thinly slice the radishes.
Warm the corn tortillas in a separate dry pan over medium heat for about 30 seconds per side until soft and pliable.
Slice the rested steak against the grain into thin strips to ensure maximum tenderness.
Divide the steak strips between the warm tortillas and top with the diced onion, cilantro, sliced radishes, and crumbled cotija cheese.
Serve immediately with a fresh squeeze of lime juice over the top.