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.
2
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.
3
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.