Preheat oven to 400. Cut defrosted halibut (or cod) into 1 1/2" strips and lay on parchment paper on a sheet pan. Drizzle olive oil over the pieces. Sprinkle salt, cumin, paprika, and chili powder over each piece, turning to cover all sides. Set aside to marinate while you prepare the rest.
Fresh Onion Salsa:
Mix ingredients in small bowl and set aside.
Taco Sauce:
Mix ingredients in small bowl and set aside.
Cabbage:
Finely chop 1/2 head of green cabbage- you can leave it plain and crunchy or mix with some of the taco sauce.
To cook halibut:
Place sheet pan in 400 oven for 10-12 minutes, until fish is starting to flake but still opaque in center.
To assemble tacos:
Heat flour or corn tortillas, place fish inside and top with cabbage, fresh sliced avocados, onion salsa, taco sauce, and cojita cheese, if desired. Serve with Mexican rice, beans, and a side of chips.