Place the rice, 2 cups (473ml) of water and 1 pinch salt in a medium saucepan and bring to boil over high heat.
Cover pan, reduce heat to low, and simmer for 20 minutes or as long as listed on the package instructions without peeking.
Remove the pan from the heat and let stand with the lid on for 15 minutes.
Add the vinegar, stir, and let stand uncovered until cool enough to handle, about 30 minutes.
Line a small (½ cup/118ml capacity) ramekin with plastic wrap, using enough that 4 inches (10cm) hang over the sides.
Scoop 2 heaping tablespoons of rice into the ramekin, top with 1 heaping teaspoon of the tuna mixture.
Top with another heaping tablespoon of the rice and spread to cover the tuna.
Bring the edges of the plastic up over the rice and twist the ends together to create a little beggar’s purse, compacting the rice into balls.
Unwrap the rice ball and place in the center of a nori piece.
Bring the sides up to cover the bottom half of the rice ball and press gently to adhere.
Repeat the process with the remaining ingredients to make 10 rice balls.
Serve the onigiri with the soy sauce within an hour or refrigerate them, tightly wrapped in plastic, for up to two days.
