Preheat oven to 425º.

Oil a heavy sheet pan with 3 tablespoons of the olive oil and place squash cut side down on pan. Roast in oven until squash is caramelized and easily pierced by a paring knife (approximately 45 minutes). Remove from oven and allow to cool.

In a heavy bottomed pot, sweat onions and celery in the remaining 3 tablespoons of olive oil until translucent. Add the sherry, Chardonnay, sherry vinegar, sugar and bouquet garni, and reduce until nearly dry. Remove pot from heat.

With a large spoon, separate squash from its thick skin and add to the pot. Return to medium heat and add just enough stock to cover. Bring to a simmer for 15 minutes. Remove from heat.

When soup base has cooled, remove bouquet garni and purée until smooth in a blender or food processor. Pass through a strainer if a finer consistency is desired.

To serve, return base to medium heat and stir in mascarpone and remaining stock to desired consistency. Season with nutmeg, salt and pepper. Soup may be garnished with roasted pumpkin seed oil and fried sage.