Preheat oven to 350°. Heat the butter in a medium pan over medium-high heat. Add the onion, celery and parsley. Cook, stirring, until the onion is translucent, about 5 minutes.
Spread the rice over the bottom of an 9 x 9" baking dish (or 11 x 7 x 2" baking dish). Sprinkle with salt and pepper and spread the onion mixture on top. Put the stock in the same pan used to cook the onion and bring to a boil. Pour it over the rice.
Cover the dish tightly with foil and bake in the bottom half of the oven for 40 to 45 minutes, until all the liquid is absorbed.