Preheat your grill to medium-high heat.
Season the flank steak with cumin, chili powder, salt, and pepper.
Grill the steak for about 5-7 minutes on each side for medium-rare.
Remove the steak from the grill and let it rest for 5 minutes.
Slice the steak thinly against the grain.
Warm the corn tortillas on the grill for about 30 seconds on each side.
Assemble the tacos by placing slices of steak on each tortilla.
Top with diced onions and chopped cilantro.
Squeeze fresh lime juice over the tacos.
Serve immediately with lime wedges and optional toppings.