Prepare Rice: Mix rice and sticky rice, wash thoroughly, and soak in water for 1 hour.
Cook Rice: Cook the rice until done, then mix hot water with salt and stir into the cooked rice.
Make Tuna Mayo Filling: In a bowl, mix tuna, sugar, pepper, sesame oil, and mayonnaise until well combined.
Shape Onigiri: Moisten hands with water, take a small amount of rice, fill with the tuna mayo mixture, and shape into onigiri. Optionally, use an onigiri mold and then wrap with plastic or nori onigiri.
Serve: Your Onigiri Tuna Mayo is ready to be served.