Heat the oil in a large, heavy-bottomed pot over medium heat. Sauté the onions, celery, carrot, and optional leek until onions are soft, about 10 minutes. Stir in the thyme and sage.
Add the butternut squash and 5 cups of the broth. (If using frozen squash, do not add yet. Reserve the final cup of broth to adjust to your desired thickness at the end.)
Bring the mixture to a boil, and then reduce the heat to a gentle simmer. Cook until the carrots and butternut squash are just tender, about 10-15 minutes.
If using frozen squash, add now that the carrots are tender. Stir in the chicken, white beans, cream or coconut milk, and cook a few minutes more or until heated through. Add the remaining one cup of broth, if desired.
Meanwhile, in a small bowl, mix the optional cornstarch with a few tablespoons of the hot broth. Add to the soup, and stir to incorporate. This will thicken the soup slightly. Add salt and pepper to taste; precise amount will depend upon type of broth (see notes) and personal preference.