Wash hands with soap and water for 20 seconds.
Preheat oven to 375°F.
Line a baking sheet with foil.
Scrub the pumpkin with a clean vegetable brush under running water. Dry the outside of the pumpkin.
Cut the top off the pumpkin; save the top. Clean out the seeds and fibrous strands. Place the pumpkin on the baking sheet.
Place the mushrooms in a colander and rinse under cool running water, while shaking the colander to move the mushrooms around and release the dirt. Slice the mushrooms; set aside.
Scrub the onion with a clean vegetable brush under running water. Dice the onion.
In a large skillet, sauté onion and mushrooms in oil.
Add ground meat and cook until no longer pink. Drain the fat from the meat.
Wash the counter and utensils that touched the raw meat. Wash hands with soap and water after handling raw meat.
Add water chestnuts, salt, and pepper.
In a large bowl, mix soup, brown sugar, and soy sauce. Add ground meat mixture and cooked rice; stir well to combine.
Empty bowl ingredients into the cleaned out pumpkin and replace pumpkin top. Bake for 1 hour and 15 minuteson 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 the pumpkin.
To serve, scoop off chunks of cooked pumpkin with the meat mixture.