Preheat the oven to 400°F. Spread the peeled, seeded, and chopped hubbard squash, sliced carrots, and chopped celery on a baking sheet. Drizzle with olive oil and toss to coat evenly. Roast for about 30-40 minutes until the vegetables are golden and tender, and the garlic has softened and begins to caramelize.
Once done, remove the roasted garlic from its skins and set aside. Transfer the roasted vegetables into a slow cooker, along with chopped onion, roasted garlic, cinnamon, and nutmeg. Pour in the vegetable broth.
Set the slow cooker to low and let the mixture simmer for 4 hours, allowing the flavors to meld and the vegetables to become very soft.
Use an immersion blender or transfer the soup in batches to a blender, blending until completely smooth and velvety. Be careful with the hot liquid!
Return the blended soup to the slow cooker (if using a blender), taste and adjust seasoning with salt and pepper as needed.
Ladle the warm, smooth soup into bowls and serve, garnished with a drizzle of olive oil or a sprinkle of fresh herbs if desired. Enjoy the comforting aroma and creamy texture.
