Make the rice: Bring 2 1/2 cups of water to a boil in a medium saucepan. Stir in the rice. Cover and reduce heat to low. Cook 10 minutes. Remove from heat.
Stir the rice vinegar, sugar, salt and sesame seeds into the hot rice.
Make the spicy tuna: Whisk together the mayo and sriracha in a large bowl. Drain the tuna and break it into small flakes. Stir the tuna into the spicy mayo.
Preheat oven to 425 degrees.
Coat an 8x8 or 9x9 baking dish with sesame oil. Press the cooked rice firmly into the dish. Lay the nori sheets on top of the rice. Spread the tuna mixture on top of the nori.
Bake for 15 minutes.
Top with diced avocado, diced cucumber and diced scallions. Drizzle yum yum sauce and additional sriracha, if desired.