In a small skillet, toast pumpkin seeds at medium heat for 8 minutes or until they release their aroma. Meanwhile, in a big casserole with boiling salted water, cook rice covered for about 1 hour or until tender and grains begin to open. Drain and set aside. In the same casserole, melt the butter at medium heat. Add onions, carrots, celery, bell pepper, salt and pepper and cook, stirring often, for 7 minutes. Add the reserved rice. Bring to desired temperature. Add grilled pumpkin seeds and stir.