Tuna Nigiri Sushi

1 (120 to 170 gm) piece sushi-grade tuna (or salmon)
3⅓ cup short-grain "sushi rice", rinsed
4 cups water
5⅓ tbsp. sushi vinegar (or prepare using 5⅓ tbsp. rice vinegar, 5 tablespoons sugar and 3 tablespoons salt)

- Wash the rice until the water rinses clear. Drain the rice in a colander and let it stand for 30 minutes. Place the rice in a rice cooker, add the 4 cups of water or follow the instructions on a rice cooker. Cooks about 15 minutes.

- Take out the bowl from the cooker and place a damp towel over the rice, and let cool for 10 minutes.

- If sushi vinegar is not available, can easily make it by pouring the rice vinegar, sugar and salt into a pan under low heat until the sugar and salt are dissolved. If prefer sweeter vinegar, add more sugar.

 - Pour the sushi vinegar over the cooked rice and mix it gently. Can do this in a wooden bowl for making sushi rice or in your rice cooker by turning off the cooker and pouring the vinegar and mixing gently and evenly and leaving it until it cools down to room temperature. The lid should remain open.

- Use the sushi-grade tuna. Slice the fish very thinly into bite-sized pieces.

- Hand-mould fingers of rice and place a thin slice of fish on the top.