Jack-o-Lantern Casserole

1 medium pumpkin
1 chopped onion
1 to 2 pounds ground beef
2 tablespoons soy sauce
2 tablespoons brown sugar
1 (4 ounce) can mushrooms (optional)
1 can cream of chicken soup
1 1/2 to 2 cups cooked rice
1 (8 ounce) can sliced water chestnuts (can
    substitute celery, chopped)

Clean out pumpkin and decorate outside with a black marker, if desired.

Brown hamburger and onion. Mix in remaining ingredients. Put pumpkin on cookie sheet. Fill pumpkin with mixture. Place in oven and bake pumpkin with lid on for 1 hour at 350 degrees F or until pumpkin is tender.