Place all ingredients, except the vinegar and mirin, in a heavy saucepan, cover and bring to a boil. Reduce the flame to low and simmer 20 minutes. Turn off the flame and let the rice sit for 10 minutes, with the lid on the pot. Remove rice with a wooden spoon or bamboo rice paddle, one spoonful at a time. Spread each spoonful evenly in the bowl. Repeat until all rice is removed. Sprinkle the vinegar and mirin over the rice and toss, gently, several times to evenly mix and to cool the rice. Let the rice sit until room temperature or refrigerate, before making sushi with it. Hot rice will cause the nori, if using, to fall apart easily.
Per serving - 308 calories, 0 g fat (1% calories from fat), 5 g protein, 69 g carbohydrate, 2 g fiber, 0 mg cholesterol, 81 mg sodium