1) If not using Kewpie mayo, mix mayo and mustard – set aside. Crush the Cap'n in a plastic bag until very fine and put on a plate.
2) Lightly oil all sides of tuna, season with salt and press down into the Cap'n making sure it's covered well on all sides.
3) Heat a non-stick pan really well then add a couple tablespoons of oil.
4) Add the tuna and sear quickly on all sides until golden brown (depending on hot your pan is, maybe 5-10 seconds a side) and remove.
5) Put a large dollop of the mayo on side of the serving plate, then spread out with the back of a spoon – then drizzle Sriracha over mayo.
6) Slice tuna into 1/2 inch pieces and place on the mayo and top with green onions. Just be sure to get some of everything in each bite.
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