Cut squash in half lengthwise; remove seeds.
Cover squash with olive oil.Sprinkle with salt, pepper, cinnamon and nutmeg. Add maple syrup and garlic cloves in squash cavity.Place in 350*oven and bake for about 50 minutes until squash is cooked.Spoon out the cooked squash and garlic; set aside.
Heat the olive oil in a large saucepan over medium heat until hot. Add the onion, celery, carrot and sauté until soft but not brown, about 10 minutes. Season with salt and pepper.
Add the chicken stock and bring to a boil. Simmer for several minutes. Stir in the squash until smooth, then simmer gently to let the flavors meld, about 10 minutes. Add a bit more maple syrup and cinnamon to taste.
Puree the soup in a blender until smooth. (The soup can be made ahead to this point, cooled, covered, and refrigerated for several days or frozen for about 1 month. It will thicken as it cools and may need thinning with stock or water when reheating.)
Return the soup to the pan and reheat gently. Add the half-and-half, if using. Adjust the seasoning with salt and pepper.
Ladle into a warm tureen, a ceramic bowl or 4 soup plates. If desired, garnish with a spoonful of sour cream and a scattering of pumpkin seeds.