you will also needa sushi mat ( or a bamboo placemat)
In a small bowl, mix together tuna, mayo, cayenne, 2 t. wasabi, salt and white pepper, and fish sauce. Set aside.
In another small bowl, mix together 1 T. sushi vinegar with a cup of water, set aside.
Spread sushi rice on a plate and sprinkle with 1 teaspoon of sushi vinegar, set aside.
Lay sushi mat on a flat work surface and place one sheet of nori, rough side up, short end closest to you.
Dip your fingers in vinegared water, pick up about a handful of rice and spread it evenly over the nori, leaving a 1 inch border at the far end of the sheet. If the rice starts to stick to you, dip your hands back in the water.
At the end closest to you, spread a few spoonfuls of the tuna mixture in a line over the rice. Place one stalk of green onion on top of the tuna mixture.
Using the mat, fold the edge of nori closest to you over the tuna filling and pull back on the mat snugly, to form a tight circle. Continue to roll the nori, using the mat, until you reach the end. Rub a dab of vinegared water over the edge of the nori to ensure that the roll seals.
Place the roll on a cutting board and cut in half. Then cut each half in half again. And then cut each piece in half once again to make 8 pieces.
Repeat those steps with the remaining sheets of nori.
Serve the rolls with pickled ginger, soy sauce and wasabi paste.