Using two separate bowls: place the diced tuna and salmon sashimi into the bowls and set aside.
In a small mixing bowl, mix together the Kewpie mayonnaise, Sriracha and lemon juice until fully combined. Taste and adjust any ingredients as needed. Divide the spicy mayo between the two bowls: diced tuna and salmon sashimi and mix together well.
For the rice:
In a medium sized bowl, add some cooked rice and add a splash of sesame oil and a splash of brown rice vinegar. Mix together. Taste and adjust any ingredients as needed. Feel free to add furikake here if desired.
For the cucumber roll:
Scoop out the cucumber flesh from both sides. Hollow out the cucumber as much as you want; keep in mind you want enough room to place the rice. You also don't want to keep the cucumber too thick; which will make it harder to eat. Feel free to cut the cucumber pieces in half again so the cucumber "roll" is about 5 to 6 inches long.
For additional flavor, splash some soy sauce or coconut aminos over the cucumbers. You can also spread some spicy mayo onto the cucumber before adding the rice. Add some of the seasoned rice into your cucumber roll and press down so the rice sticks to the cucumber as much as possible. Alternatively, you can also splash some soy sauce or coconut aminos over the rice. Add the spicy tuna or spicy salmon over the top. Garnish with diced green onions. Feel free to sprinkle on some furikake over the top if desired. To make these easier to eat, you can slice into smaller pieces. Enjoy!