Heat oven to 400 F. Lightly brush the cut sides of the squash with some of the sunflower oil, salt, & pepper. Place cut-side-down on a baking sheet. Place the garlic bulb on a piece of aluminum foil and drizzle some of the sunflower oil on top. Wrap the foil up and around the garlic bulb to form a pouch. Place the pouch on the baking sheet with the squash, then bake for 35-40 minutes or until squash is fork-tender. Set aside to cool. Heat the remaining oil in a large soup pot. Add the onion, carrots, & celery and cook over medium-high heat, stirring frequently, for about 8 minutes. Meanwhile, peel the squash and mash it. Add the stock to the soup pot, then add the squash and squeeze the roasted garlic into the pot, discarding the remaining garlic skins & base. Season well with salt & pepper. Mix well and bring to a gentle boil, then reduce heat to simmer for about 10 minutes. Use an immersion blender to puree the soup, or work in batches in a blender or food processor, returning the pureed soup to the pot over low heat. Add the milk or cream and stir, then add the splash of vinegar. Serve with a dollop of yogurt or sour cream. For a spicy kick, try adding a dash of VT Maple Sriracha.