Rinse the rice under cool water until the water runs clear. Bring 4 cups or water to a boil, add rinsed rice and bring back to a boil. Reduce heat to a simmer, cover and let cook for 18 minutes, or until water is absorbed and rice is tender. Fluff rice with fork and spread out in a 9x13" casserole dish to let cool slightly.
For The Sauce
Combine the sauce ingredients - kewpie mayo, sriracha, sesame oil, salt and white pepper in large bowl. Whisk to combine. Add in the cubed ahi tuna (raw), cucumber, green onions and 1 tbsp of the tobiko eggs (optional). Gently fold together, so that it is all coated in the sauce.
Scoop and spread the tuna mixture over the slightly cooled rice into an even layer.
Drizzle more kewpie mayo over the top, followed by more sriracha if desired. Sprinkle on the remaining 1 tbsp of tobiko eggs and furikake. Enjoy right away!