Prepare rice according to package directions.
Preheat oven to 350°F. Grease a 13x9-inch baking dish.
Place chicken in the bottom of prepared casserole.
In a bowl, combine cooked rice with celery, tomato, onion, mayonnaise, pecans, salt, pepper, and nutmeg. Spoon rice mixture around chicken. Sprinkle chicken and rice with cheese. Sprinkle with bread crumbs, if using. Cover and bake for 30 minutes. Remove cover and bake for 15 minutes more, or until cheese is bubbly and chicken is cooked through.