Rinse rice in cold water 4-5 times until water runs VERY clear.
Combine rinsed rice, 4 cups cold water, and mirin in a pot and let soak for 30 minutes.
Bring rice to a boil over medium-high heat; immediately turn heat to low, and cover. Cook 10 minutes, remove from heat and let sit, covered, 15 minutes.
Raking rice with rubber spatula - Pour rice vinegar evenly over rice and fold gently with a rubber spatula to mix. Leave rice in front of a fan, or fan with a piece of cardboard for 15 minutes or until rice is just above room temperature.
Heat 1 tablespoon sesame oil in a large cast-iron or non-stick saute pan over medium heat. Add rice; pat out in an even layer. Cook, rotating skillet for even browning (do not stir), until rice is golden and crisp on bottom, about 15 minutes.