Slice hamachi into thin sashimi slices so that they will arrange to cover the upper outer surface of the sushi roll. About 3-4 slices depend on your hamachi block size.
Mince the remain hamachi finely but do not ground so that you still have a bit of texture.
Finely chop the green part of green onion/scallion.
In a small bowl, combine diced hamachi, scallion, Sriracha sauce, and 1/2 Tbsp spicy mayo.
Cut mini cucumber into thin strips.
You will need a plastic wrap (to help hold/shape this roll plus avoid sticking) and a small cup of vinegar + water to moist your hand in between.
Lay a 1/2 sheet of nori on top on the plastic wrap, texture side up.
Spread and knead 1/2 cup of sushi rice onto nori. Do not mush the rice.
Flip the nori over, so that the rice layer directly on plastic wrap.
Spoon negi hamachi (spicy yellowtail mixture above) on the nori.
Arrange cucumber onto negi hamachi.
With the help of a towel, lift rice/nori up to cover the ingredients, lightly press and hold for a second to shape then roll.
Keep rolling till the end, hold and shape.
Now carefully open the plastic wrap to reveal the roll.
Drip a small straight line of spicy mayo on top the roll for extra flavor and adhere the fish.
(optional) Dab/drip some wasabi next to the line of spicy mayo.
Arrange the sliced hamachi on to cover top of the roll.
Tuck and wrap the plastic wrap. Lightly press and hold for a couple seconds.
Keep a damp towel to wipe clean your knife in between, cut the sushi roll into pieces while plastic wrap still on.
Carefully unwrap each piece and arrange onto serving plate.