Mix together all sauce/marinade ingredients and stir together until fully combined.
Pour half of the sauce into a ziplock bag or air-tight container. Place the tuna steaks inside. Marinate in the fridge for at least 20-minutes and up to overnight. Leave the other half of the sauce for pouring over the completed tacos.
While the ahi tuna marinates, put together the slaw. Add all cabbage-carrot slaw ingredients into a large bowl. Mix thoroughly and leave to the side or in the fridge up to overnight.
When ready to prepare the tacos, heat the cast iron or pan on the stove with the oil. Let the pan get hot (about 2-4 minutes).
Take the tuna steaks directly from marinating onto the scorching hot pan, large flat side down. (Make sure to have a vent fan on, it helps!)
Let tuna steaks sear for 1-minute on the original side. Using tongs, flip to the second side and let cook an additional 1-minute. Turn off heat.
Remove seared ahi tuna steaks from the cast iron pan, and place onto a cutting board. Allow to sit for 5-minutes.
While tuna steaks rest, heat the corn tortillas in a large pan on the stove at medium-heat. Let warm for 1-minute each side and then stack on a plate lined with paper towels or regular towel.
Slice the tuna steaks in ½ inch thick slices.