Preheat oven to 425 degrees. In a medium saucepan, whisk together miso and 2 ¼ cups of water until dissolved. Add rice, bring to a boil, then cover, reduce heat to low, and simmer until all liquid is absorbed and rice is tender, about 15 minutes. Remove from heat and let stand for 5 minutes. Fluff rice with a fork (it will be slightly sticky).