Pat the flank steak dry with paper towels and rub both sides with olive oil, cumin, chili powder, sea salt, and black pepper.
Preheat a grill or cast-iron skillet over medium-high heat until very hot.
Place the steak on the grill and cook for 4-5 minutes per side for medium-rare, or until it reaches your desired doneness.
Remove the steak from the heat and let it rest on a cutting board for at least 5 minutes to keep the juices intact.
While the steak rests, prepare the crema by mashing the avocado in a small bowl with the Greek yogurt, lime juice, and finely chopped cilantro until smooth.
Thinly shred the red cabbage and slice the radishes into paper-thin rounds.
Warm the corn tortillas on the grill or in a dry pan for about 30 seconds per side until pliable.
Slice the rested steak against the grain into thin strips.
Assemble the tacos by layering the steak into the warm tortillas, then topping with shredded cabbage, radish slices, and a dollop of avocado crema.