In a heavy-bottomed saucepan, add the sushi rice and water and bring to a boil. Once boiling immediately cover the pan with a lid and reduce the heat to a simmer. Allow to cook for 20 minutes. Turn off heat and leave pot covered on stove for an additional 10 minutes. Remove rice from pot and allow to cool slightly.
In a small bowl, combine vinegar, salt and sugar and whisk until completely dissolved. Add to the rice and coat evenly.
Lay the nori sheet on top of a bamboo sushi mat. Cover the surface of the sheet with partially cooled sushi rice, making sure to leave a thin strip at the top to allow for an even seal. Place the cucumber, tuna and avocado just below the midway point of the nori and tightly wrap the rice in a spiral to form the sushi roll.
Cut a piece of nori into a triangle to form the base of the pizza. Place the sushi roll crust on the top of the triangle to create a pizza slice. Cover the nori with an even layer of rice, then top with salmon, ahi tuna, avocado and nori flakes.