Cook the rice. Rinse the rice under cold water until the water runs clear to remove any excess starch. In a medium saucepan, bring the rice and water to a boil, cover, and cook for 20 minutes over medium-low heat. Remove from the heat to steam for an additional ten minutes. You can also use a rice cooker or make instant pot sushi rice. Season and chill. In a small microwave-safe bowl, whisk together the rice vinegar, sugar, and salt and gently heat in the microwave for 15 seconds to help the sugar dissolve then stir to combine. Pour over the warm cooked rice and use a "cutting method" with a rubber spatula or rice paddle to mix the rice until the seasoning is distributed.
Shape the rice. Tightly pack the seasoned rice into a small shallow baking tray lined with plastic wrap or parchment paper. Place it in the fridge to cool completely. It should be about 3/4-1 inch tall. With a sharp knife, cut the rice into bite-sized rectangles about 2 inches long. You may need to wipe down the blade with a wet cloth between cuts to keep the rice from sticking to the knife.
Crisp it up. Add a thin layer of neutral oil (such as avocado or refined coconut oil) to cover the bottom of a large frying pan or skillet over medium to medium-high heat. Cook the rice patties on each side for 4-5 minutes or until golden brown. Blot the excess oil off on a paper towel and transfer the crispy rice squares to a cooling rack to allow them to further crisp up.
Make spicy tuna mixture. While the rice squares cool, add the minced tuna, spicy mayo, and additional sriracha (if desired) to a bowl and mix to combine. Set aside.
Assemble. To the top of the crispy rice squares, add avocado slices, followed by 1 large tablespoon of the spicy tuna mixture, a drizzle of spicy mayo, and a garnish such as chopped chives, green onions, or a sprinkle of sesame seeds. Serve with low-sodium tamari or soy sauce.