Add the seasoned rice vinegar to the cooked sushi rice and mix together. Place your bamboo rolling mat inside a plastic gallon bag (trust us, this makes clean up much easier, all you have to do is throw out the bag!)
Place one half sheet of nori on the rolling mat (remember we tore the full sheet in half hamburger style) with the rough side facing up. ONLY ONCE THE RICE HAS COOLED, grab a handful of rice and cover the rough side of the nori leaving a 1 inch gap at the top without any rice. If using sesame seeds, sprinkle them over the rice. Grab the nori by the top, and flip it over so the rice is against the bamboo rolling mat and the 1 inch gap is now on the bottom.
Starting 1/3 of the way up from the bottom begin to align the "guts" of the sushi roll in a horizontal line. Begin with the cucumber matchsticks and align them across the nori. Next, take your shredded imitation crab meat and align it above the cucumber. The crab should be about a half inch to an inch thick all the way across. Alternatively, just using the whole crab sticks works just as well.
Grab both the sushi mat and the nori and roll the mat over from the bottom until the empty space we left at the bottom is touching the other side, squeezing down on the roll to make it nice and tight and allowing to hold its shape.
Now that the roll is formed, it is now time to add the "Rainbow"! Cut the salmon, tuna, and avocado lengthwise and trim each one until they are no thicker than a quarter inch. Layer the tuna, salmon, and avocado alternatively across the top of the roll like a blanket to where it looks like a rainbow!
Once layered, it is a good idea to wrap the roll in cling wrap and roll it wound on your counter or cutting board. This helps the meats on top of your roll stay formed and in place.
Place the roll on a cutting board and get a knife and damp paper towel. Cut the toll into 8 equal sizes (easiest if you start by cutting the roll in half). Before you make each cut DAMPEN THE KNIFE WITH THE PAPER TOWEL or else the roll will fall apart when you slice through it.
Serve with soy sauce, take some pictures for your friends, and enjoy!