Cut squash in half, remove seeds and place squash face down on an oiled baking sheet. Bake about 30-45 minutes (depending upon size) in a preheated 350 degree F oven until soft.
While squash is baking, heat the butter in a heavy soup pot over medium low heat. Add onions, leeks, and carrots. Sweat until soft. Do not brown. Add vegetable stock to soup pot and bring to a gentle simmer.
When squash is done, scoop cooked squash from the skin. Add squash to the soup pot. Continue to simmer about 30 minutes, or until vegetables are thoroughly cooked and softened.
Puree soup mixture with an immersion blender. If you do not have one you can also do this in a blender or food processor. This may require several batches. Strain soup through a medium mesh strainer and discard the solids.
Season with allspice, ginger, salt and pepper. Portion soup into individual bowls.