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 seeds and fibrous strands. Place the pumpkin on the baking sheet. Save the top.
In a large skillet, sauté onion and mushrooms in oil. Add ground meat and cook until no longer pink. Drain the grease from the meat. Add seasonings and water chestnuts.
In a large bowl, mix soup, brown sugar, and soy sauce. Add ground meat mixture and cooked rice.
Empty bowl into the cleaned out 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 scooped off the sides with a spoon. Cooking time will vary depending on the size of your pumpkin.
To serve, scoop off chunks of cooked pumpkin with the meat mixture.