In a small skillet, grill pumpkin seeds at medium heat for 8 minutes or until they begin to smell. (You can do this step earlier and put in an hermetic container. They will preserve up to 24 hours at room temperature).
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. (you can do the rice, let cool and cover until next day. Add 2 tablespoon of water before to reheat.) Reheat. Add grilled pumpkin seeds and stir.