Cut the squash in half and scrape out the seeds. Place it cut-side up in a baking pan with about 1/4 inch of water. Cover with foil and bake at 375 F for about 45 minutes, or until soft. Allow to cool completely. I usually do this step early in the day.
In a large soup pot heat the oil. Add the leeks, celery and ginger and cook until soft but not browned - five to ten minutes.
Peel the cooled squash then cut into pieces and add to the soup pot. Pour in 4 of the cups of stock. Bring to a boil, then cover and simmer for 20 minutes, stirring occasionally.
Add the tofu to the pot and then puree the soup using a blender or food processor. Return to heat add remaining 2 cups of stock and the salt. Stir thoroughly.