Roast the squash in a 375 degree oven until tender. (See this post for detailed directions.) This will yield approximately 4 1/2 cups of mashed squash.
In a skillet, saute the onion and celery in the olive oil over medium low heat until tender. Add the garlic and saute for another 30 seconds.
Add the roasted squash, broth, and maple syrup. Then add the cinnamon, cumin, ginger, and cayenne pepper (if using). Stir everything together, then simmer for 20 to 30 minutes, until thickened to your liking.
Taste the soup and add the teaspoon of salt (or more) if needed. Adjust the other seasonings as needed.
Use an immersion blender to make a smooth pureed soup. If you don’t have an immersion blender, use a regular blender or food processor to puree the soup in small batches.