Heat the oven to 425°F and line a sheet pan with parchment paper.
Pat the cod dry, set it on the pan, and rub with 1 tbsp olive oil, the cumin, chili powder and salt. Bake 12-15 minutes, until the fish is opaque and flakes easily with a fork (145°F at the center).
While the cod bakes, toss the cabbage with the Greek yogurt, lime juice, remaining 1 tbsp olive oil, cilantro and a pinch of salt. Let the slaw sit 10 minutes so it softens slightly.
Warm the tortillas in a dry skillet for about 30 seconds per side, or wrap them in foil and put them in the oven for the last 5 minutes.
Flake the cod into large pieces, fill each tortilla, and top with the cabbage slaw and avocado slices. Serve right away.