With a sharp knife, prick the squash in several places to allow steam to escape while it cooks
(otherwise it could burst). Place the squash in a baking dish lined with foil and roast for about 90 minutes, until the squash is soft when you push on it. Let cool for 30 minutes, so it=s easier to handle, then peel, seed, and remove strings. Cut the roasted squash into 1/2-inch pieces, or scoop meat out with a spoon.
In a heavy soup kettle, heat the olive oil and add the onions. Cook until light golden brown and add the leeks and cumin. Cook for 2 minutes and add the garlic and ginger. When the garlic is fragrant, add the maple syrup, soy sauce, bourbon, sherry and nutmeg. Add the squash and stock and bring to a boil. Lower the heat and cook gently for 15 minutes.
Using an immersion blender (or a regular blender), puree the soup until very smooth. Add the evaporated milk, salt, and pepper. Cook for 2 minutes; do not bring to a boil. Add the cornstarch mixture and stir until thickened, about 5 minutes. Serve in soup plates.
If you like, garnish this soup with garlic and Parmesan cheese croutons and a dollop of non fat sour cream.