Koshihikari rice is exceptionally flavorful, tender, and just lightly sticky. Mushrooms bring the right balance of umami, without overwhelming the flavor of the rice itself.
Place rice in a fine wire-mesh strainer, and rinse under cold water until water runs clear. Transfer to a paper towel–lined baking sheet, and pat dry.
Combine rice, mushrooms, dashi, mirin, and soy sauce in a medium pot; let stand 30 minutes. Bring to a boil over medium-high; reduce heat to low, cover, and simmer 15 minutes. Remove from heat, and let stand, covered, 15 minutes before serving.