Slice the squash in half long ways and scoop out the seeds. Place the squash cut side down on a baking sheet. If you line the baking sheet with foil or parchment paper the clean up will be easier.
Bake for 30 – 45 minutes or until the thick part of the squash can easily be pierced with a fork. Let cool then scoop out the pulp and discard the skin.
Note: This step can be done in advance.
In a large soup pot over medium-low heat, melt the butter. Add the chopped leeks and ginger. Cook for 5 – 8 minutes until tender, but not browned.
Add the squash pulp to the pot along with the 5 cups of chicken stock (I recommend homemade). Bring to a simmer and cook, stirring and breaking up the squash with the back of your spoon, for 10 – 15 minutes.
Puree the soup either with a hand immersion blender or in a counter top blender. I highly recommend a hand immersion blender if you like to make soups because you’ll end up with a lot fewer dirty dishes.
After the soup has been pureed, stir in the cream and salt (to taste).
Ladle into soup bowls and serve warm topped with chopped sage and pistachios. Refrigerate or freeze the leftovers.