Combine avocado, salt, pepper, and red pepper flakes and mash/stir to combine. I like mine a little chunky still for the texture. Set aside.
Use a sharp knife to finely chop/mince the tuna filet.
Add minced tuna, mayonnaise, soy sauce, sriracha, and sesame oil to a medium sized bowl. Stir to combine. Taste and adjust seasoning to taste. Store in the fridge covered while cooking the rice.
Take your pre-cooked white rice (I think leftover rice works great and sticks together well). Cup about a 1 ½ inch round ball of rice in the palm of your hands. Gently ball it together like you would pack a snowball. Once well rounded, put the rice ball on a plate and gently press down to form a disc shape. Repeat until all of the rice is used, you should have about 12 in total.
Add vegetable oil to a medium skillet and heat on medium-high heat. Once the oil is shimmering, add the rice and fry until crispy and browned, about 4 minutes. Flip and repeat on the other side. Once fried, gently remove from pan and let sit on paper towels until ready to assemble.
Add crispy rice to a plate of platter, then top with mashed avocado. Divide equally among all of the rice. Add prepared spicy tuna on top of the avocado. I like to gently press it into the avocado. It works as a sort of "glue" and helps the tuna not slide off. Once assembled you can top with additional optional add-ons like sesame seeds, nori flakes, or sprouts. Enjoy!
Rice NoteI think leftover white rice works great for this recipe. It sticks together well and also fries easily. I do not recommend using brown rice.Leftover StorageI do not recommend storing leftovers. The sushi should be consumed immediately and leftover should be disposed of.Oil NoteMake sure to add enough oil so the entire bottom is coated in a thin layer. You don't want to submerge the rice completely, just the bottom should be fried. This makes it so the outside is crisp and crunchy and the inside is still soft.