Rinse the rice thoroughly. Place in a large saucepan and add enough water to cover by several inches. Bring to a boil, then remove from heat. Cover and set aside for 1 hour. Drain.
Preheat the oven to 350°F. Grease a 2-quart casserole. In a large skillet, melt the butter, add the mushrooms and onion, and sauté for 8 minutes, or until browned. Add the rice and remaining ingredients to the skillet and mix well. Spoon the mixture into the casserole and bake for 1 hour, or until the rice is tender.