Step 1: Cook the RiceWash: Rinse the rice in cold water 3 to 4 times until the water runs completely clear, then drain.Soak: Let the drained rice sit in a colander for 30 minutes before cooking.Cook: Put rice and 2.5 cups of water in a pot. Bring to a boil, then cover with a lid, turn heat to low, and simmer for 12-15 minutes.Steam: Take the pot off the heat. Keep the lid on and let it steam for 10 minutes. Fluff with a spatula and let it cool slightly until warm.Step 2: Make the FillingTuna Mayo: Mix the drained tuna, Kewpie mayo, soy sauce, and black pepper in a small bowl until smooth.Teriyaki Chicken: Stir chicken, teriyaki sauce, and ginger in a pan over low heat for 2 minutes. Stir in green onions and let it cool.Step 3: Shape and WrapPrep Hands: Wet your hands with water and rub a pinch of sea salt across your palms.Fill: Take $1/3$ cup of warm rice in your hand, make a small well in the center, and add 1-2 teaspoons of filling.Shape: Fold the rice over the filling. Cup your hands together to gently mold the rice into a tight triangle shape.Wrap: Stick a strip of nori seaweed at the bottom of the triangle. Serve immediately or wrap in plastic cling wrap for later.