Toss the cabbage with 1/4 teaspoon salt in a medium bowl and set aside.
Stir the yogurt, lime zest, lime juice and cayenne pepper together in a small bowl and refrigerate until ready to use.
For the fish:
Brush the fillets all over with oil and season generously with salt and pepper and then with the spice rub. Lay the fish on the grill and cook, undisturbed, until you see distinct grill marks and can lift the fish without it sticking, 5 to 6 minutes. Test it by gently lifting a corner?if it sticks, cook it a bit longer. Turn the fish over and cook until firm to the touch, about 5 minutes more.
Microwave or steam tortillas until warm, or grill until slightly charred but still pliable. Fill each tortilla with 1/2 a fish filet, a sprig of cilantro and some cabbage. Drizzle with the yogurt sauce and hot sauce if using. Serve with lime wedges.
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