Preheat the oven to 350 degrees F.
Over moderately-high heat in an ovenproof saucepan, melt the butter until the foam subsides. Add the mushrooms and saute until golden brown, 4 to 5 minutes. Add the garlic and stir until fragrant, about 1 minute. Add the rice and stir constantly until the rice is coated and lightly toasted. Season with salt and pepper. Add the chicken stock and bring up to a boil. Cover with a tight fitting lid and cook on the middle rack in the oven until all the liquid is absorbed and the rice is tender, 17 to 20 minutes. Remove from the oven and let stand, covered, 5 minutes. Fluff with a fork before serving.
BYOC: Want to get more whole grains in the kids? Try doing this with brown rice and add a little more chicken stock and a little more time.