Preheat oven to 375 degrees.
Line a baking sheet with foil.
Thoroughly wash and dry the outside of the pumpkin.
Cut the top off the pumpkin and clean out the pulp and seeds. Place the pumpkin on the baking sheet. Save the top.
In a large skillet, sauté onion and mushrooms in oil or butter. Add ground beef and cook until no longer pink. Drain the grease from the beef. Add salt, pepper, and water chestnuts.
In a large bowl, mix soup, brown sugar, and soy sauce. Add ground beef mixture and cooked rice.
Empty the bowl into the cleaned out the pumpkin and replace pumpkin top.
Bake for 1 hour on the lower rack of the oven, or on the rack where the pumpkin can be most centered in the oven.
After 1 hour and 15 minutes, remove the top and check the sides of the pumpkin for doneness. The outside of the pumpkin will turn a dark orange, and the inside of the pumpkin should be tender and easily scoop off the sides with a spoon. If the inside of the pumpkin is still not fully cooked, bake for an additional 10-15 minutes. The cooking time will vary depending on the size of the pumpkin. Sometimes I've cooked the pumpkin as long as an hour and a half before it is ready. Using hot pads, place the pumpkin onto a serving plate, remove the lid, and stir. Be sure to scoop off chunks of cooked pumpkin into the casserole as it is served. That's the best part!