Heat the olive oil over medium-high heat in a large soup pot Add the onion, carrots and celery and saute until tender, about 5 to 10 minutes. Add squash, water, broth, brown sugar, salt, cinnamon and nutmeg. Let soup simmer for about 15 minutes. Remove from heat. Place soup in a blender, small amounts at a time. Place a towel over the lid of the blender to avoid the lid popping up. Add blended soup back to soup pot and bring to a boil. In a small bowl, combine the cornstarch and water to make a slurry. Add 1 tablespoon of cornstarch slurry at a time to the boiling soup mixture and whisk until the soup reaches desired thickness. Add half-and-half, stir and serve.