Preheat oven to 425º and line a sheet pan with parchment paper.
Slice the squash in half lengthwise and discard the seeds. Rub 1 Tbsp of olive oil over each squash and place cut side down on the pan. Roast until squash is tender and easily pierced with a knife, about 45 minutes. Remove from the oven and cool. Once cool to the touch, separate the squash from it’s skin by removing it with a large spoon and set aside until needed.
In a heavy bottomed pot over medium-low heat, sauté onions and celery in the remaining olive oil until translucent. Add sherry, Chardonnay, sherry vinegar, and the bouquet garni and reduce until nearly dry. Add the roasted squash and 4 cups of stock and bring to a simmer over medium heat. Simmer for 15 minutes. Remove the bouquet garni.
Transfer the soup base to a blender and puree until smooth. Return the soup to the pot over medium heat and incorporate butter and remaining stock 1 cup at a time until the desired consistency is reached. Add chopped sage, nutmeg, pepper, and salt (add more salt to taste if needed). Remove from heat and serve.
Garnish each serving with a spoonful of walnuts and a drizzle of sherry vinegar and pumpkin seed oil.