Soak the potato slices in a bowl of cold water for 10 minutes. Drain well and lay them out on a paper towel to dry.
Pat the ahi tuna steaks dry with a paper towel.
Mix half of the olive oil, lime juice, and cajun seasoning in a bowl. Coat the ahi tuna steaks completely. Allow them to sit in the marinade until you are ready to cook them.
In a nonstick pan heat the vegetable oil on medium heat until its hot. Fry the potato slices in batches in a single layer until both sides are golden brown. Once golden brown and crisp space them out on a paper towel and season with salt and pepper.
Heat a non-stick pan on medium-high heat until very hot.
Add the remaining olive oil to the hot pan. Sear the tuna for 1 minute on each side for rare; 2 minutes on each side for medium-rare; or 3 minutes on each side for medium. (If desiring medium-well or well-done sear the outside of the tuna in the pan 1 minute per each side and finish in a 350-degree oven on a baking sheet for 5 - 6 minutes so the tuna does not burn)
Remove the tuna from the pan to a cutting board and allow to rest for 3 minutes. Slice and garnish with cilantro. Serve with fried potato chips, pickled cucumber (see recipe), and carrot ginger puree (see recipe).