Wash and pat dry tuna with kitchen towel/paper.
Prepare two medium bowls: one with soy sauce, sesame oil and the other with mixed black and white sesame seeds.
Add the tuna to the marinade first, make sure it is coated on all sides. Let marinate for 10-20 minutes. Dip the tuna into sesame seeds until all the sides are covered. You may need to press the seeds into the tuna to make sure they stick nicely.
Place a pan over medium heat and add enough oil.
Once the oil is hot add tuna steak and sear for 2 minutes on one side. Flip the tuna and sear the other side for another 2-3 minutes. Important to note that the cooking time will depend on the thickness of the tuna steak. Mine was about 1 ½ inches thick. Place a good nonstick skillet/pan over medium heat and add enough oil.
For more traditional 4-6 ounces and 1 ½ inches thick tuna steak. Sear for 1 ½ minutes on one side, flip and sear another 1 - 1 ½ minutes. Orr use a thermometer to register 118-120 Fahrenheit in the center for medium-rare (that's what i did).
Allow resting on a plate for 1-2 minutes. Once it's cooled more it will be easier to cut it into even slices. Season with sea salt and black pepper.
Ahi tuna is ideally served medium-rare but if you prefer your tuna more cooked, add another 1-2 minutes per side. Just make sure not to overcook it, it should still be pink inside.
If not sure, take it off the pan, cut in the middle, and see if it is cooked to your liking, if not just sear it for a couple of minutes longer. You only need a minute or two here for a medium-rare tuna steak. If all the pink is disappeared, the tuna is still edible but will be more on the dryer side.
Important to note here: different burners will give different results but use your best judgment and always aim for ''not overcooking'' the tuna steak. In this case, you can still fix the situation.
Once you start cutting it up into slices and see that the steak seems too raw for you, just pop it back into the hot pan and cook for a minute or so. But if overcooked there's nothing you can do anymore.
Meanwhile, warm up your tortillas on a hot dry pan until golden brown or heat in the oven. Or on the grill!
Assemble. In warmed taco shell add a handful of shredded cabbage, 1-2 tablespoons of Mango Habanero Salsa, and top with or two slices of fish.
Drizzle with fresh lime juice, fresh black cracked pepper, and sea salt flakes, and serve these mouthwatering seared ahi tuna tacos immediately.