Measure & level 1 scoop of rice (approx. 6 ounces or ¾ cup rice if using standard English measuring cup). Transfer the rice to the rice cooker’s inner cooking pan. To rinse the rice, cover the rice with warm water, give it a good jostle with your hands to agitate the dust off the rice’s surface, then carefully drain the water. Repeat rinsing until the water runs clear – this takes a good minute or two; be patient!
Following the ratios provided in manufacturer’s directions, add water to the inner cooking pan. Place the inner cooking pan into the rice cooker & cook according to manufacturer’s directions. Once the rice is done, let it steam & rest for 5-10 minutes before seasoning the sushi rice.
Combine Japanese rice vinegar, mirin, white sugar, salt, & msg (if using) in a small saucepan over medium-high heat. Bring mixture to a boil & cook until the sugar is dissolved. Remove from the heat & set aside to cool. Once the rice is cooked, use a plastic rice paddle to transfer it to a shallow pan. Working a few spoonfuls at a time, add the prepared seasoning liquid to the rice & use the rice paddle to gently toss the seasoning throughout the rice. Use only enough seasoning to flavor the rice to taste – too much seasoning will cause the rice to be wet & mushy.
Meanwhile, as the rice cooks, prepare the spicy tuna. To a medium bowl, add the mayonnaise, sriracha, soy sauce, toasted sesame oil, & white pepper. Mix to combine well. Add the cubed tuna then gently stir to coat well. Set aside for sushi stack assembly.
Place a sushi stack mold on a serving plate or prepared surface. Gently add about ½ cup of the seasoned sushi rice into the mold, using a fork to spread it into an even layer without pressing down on the rice. Gently spread 1 heaping Tbsp smashed avocado over top, then layer on 1 heaping Tbsp of diced cucumber. Finish with 2 heaping Tbsp of spicy tuna, using a fork to spread it into an even layer. Use the press to lightly press down on the tuna &, at the same time, slide the ring up & off, revealing a beautifully layered sushi stack.