"Onigiri is a Japanese food made from white rice formed into triangular or oval shapes and often wrapped in nori (seaweed). This is just a simple one for people who may not have a lot of time, I use a rice cooker but I will go about as if I didn't."
Place rice in a bowl with fresh water. Stir until water becomes cloudy; drain and rinse. Repeat with fresh water until water no longer clouds, 2 or 3 times more.
Bring 1 1/4 cups water, rice, and salt to a boil in a saucepan; reduce heat to low. Cover and simmer, lifting lid no more than once, until water is absorbed, about 20 minutes. Remove from heat and set aside to finish cooking, about 10 minutes more. Remove cover and cool rice until no longer hot to the touch, about 15 minutes.
Mix tuna, mayonnaise, and pepper together in a bowl until no large chunks remain. Arrange nori strips on a serving platter.
Lay a 10-inch sheet of plastic wrap on a work surface. Place 1/2 cup cooked rice in the center, form a well in the middle, and fill well with 1 teaspoon tuna mixture. Top with another 1/2 cup rice. Gather edges of plastic wrap together over rice mixture, twist together to tighten, and shape rice mixture into a pyramid shape. Remove plastic wrap and gently place onto nori strip, with nori edges protruding (for use as handles when eating). Repeat with remaining rice and tuna mixture.