Preheat oven to 375º F.
Cut off the top of the pumpkin and thoroughly clean out the seeds and pulp. Rinse well and pat the inside dry.
In a large skillet, sauté the onions in oil until tender.
Add the meat and brown. Drain the drippings ,if any, from the skillet.
Add the soy sauce, brown sugar (if using), mushrooms and soup. Simmer 10 minutes, stirring occasionally.
Add the cooked rice and water chestnuts. Stir to combine.
Spoon the mixture into the cleaned pumpkin shell. Replace the pumpkin top and place the filled pumpkin in a shallow cake pan or on a rimmed baking sheet which has been lined with foil.
Bake for 1 hour, or until the flesh of the pumpkin is tender and easily pierced with a fork. Depending on the size of the pumpkin - this could take up to 2 hours.
Put the pumpkin on a platter. Remove lid and serve the meat/rice filling. Don't forget to scoop out some of the cooked pumpkin to eat as well.