In a medium-sized bowl, combine tuna, Japanese mayo, soy sauce, sesame oil, rice vinegar, & sriracha (if using). Set aside.
In the onigiri mold, sprinkle a pinch of salt, then fill the mold about halfway with rice. Dip your fingers/spoon with some water to help prevent sticking.
Scoop tuna filling to the center, then add another layer of rice to fill the mold. Sprinkle a tiny bit more salt on top.
Cover with the mold's top & firmly press down. You should feel a bit of resistance, otherwise, add more rice so the onigiri is packed.
Press the back of the onigiri mold to release the onigiri. For extra flavor & texture, roll the onigiri in black sesame seeds (this is optional!)
Wrap the onigiri with a nori sheet, then repeat the process for remaining rice & filling. Enjoy immediately!
Enjoy your spicy tuna onigiri immediately or pack it up to enjoy on the go! It's perfect for long road trips, picnics, or workweek lunches - no utensils needed!
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