Fillet the Jack if small enough; otherwise, use a hacksaw to cut steaks 3/4 of an inch thick. Place in a freezer bag after barely covering the fish with buttermilk. Get as much air out of the bag as possible and refrigerate for 4 to 8 hours.
Take the Jack out of the bag. The buttermilk will be dark pink and the Jack will be about the same shade. Don't dry it. Put it in a shallow baking pan and bake it at 350 degrees for 30 minutes. Don't bother to preheat. When the 30 minutes are up, remove all bones and skin. Chop the meat.
Dampen your tortillas with a little water and heat in the microwave.
Lay all the ingredients out and let people build their own. Feel free to add guacamole or other yummy additions.
The number of people you serve will depend on the size of the fish. Leftovers are great!