Mix together lime juice, tequila, mexican oregano, minced garlic, onion powder, ground cumin, and 1 tablespoon of fresh cilantro in a small bowl.
Place tuna fillet in a ziploc bag and pour marinade over fish.
Remove excess air from bag and seal.
Refrigerate for one hour, while you prepare your grill.
Prepare grill by spraying grill surface with non-stick cooking spray BEFORE applying heat.
Over medium-hot grill (gas, charcoal, electric) cook tuna, turning once, until cooked through (you may just cook on exterior if you prefer).
(The cooking time will depend upon the thickness of your fillet; tuna can be eaten rare. I suggest a fillet that is slightly pink in the center so it flakes easily without being overdone. Do not overcook your fillet, however.) Set aside.
Briefly heat corn tortillas carefully on grill.
Do not overcook, or they will get hard.
Break tuna into small chunks or shredded pieces and place about 1 1/2 ounces on a double layer of corn tortilla for each taco.
Add shredded cabbage and chopped cilantro and squeeze a wedge of lime over the taco contents.