Instead of searing the tuna "tataki" style, I fried it. I left the inside a little rare, like tataki tuna, but cook it to your desired degree of done-ness. As the tartar sauce goes well on both rare and well-done cuts, I used yoghurt to reduce the sourness and give a mild taste.
Prepare the tartar sauce: Finely mince the onion, and soak in water for about 10 minutes. Separate the egg white from the yolk of the hard boiled egg, finely mince the egg white, and mash the yolk.
Combine thoroughly drained onion, egg white, egg yolk, minced cucumber, mayonnaise, yoghurt, and lemon juice. Sprinkle black pepper, parsley, and basil to taste.
Prepare the deep fried skipjack tuna: Cut slab of tuna into 100 g per person, coat in flour, dust off excess flour, dredge in whisked egg, and panko.
Deep fry tuna in oil heated to 360-375°F/180-190℃ until golden brown.
Slice into bite-sized pieces, arrange on plate, and coat in tartar sauce.
Story Behind this Recipe
Since I always eat slapjack tuna seared, I wanted to try a different way of preparing it.
If deep-fried at a high temperature, the batter will turn golden brown and the tuna will be rare. For those who prefer their tuna well done, allow time for the residual heat to cook the insides. Or, reduce the temperature of the oil a bit to cook through to the tuna with a longer frying time. I've added yoghurt to the tartar sauce instead of pickles to reduce the sweetness. Also, there is no problem in omitting the lemon juice.