Preheat your oven to 400F. Line a baking sheet with aluminum foil.
Halve the squash lengthwise and discard the seeds and strings. Drizzle the inside of the squash with olive oil. Rub with the brown sugar, chili powder, and salt and pepper to taste.
Halve the onion and slice the garlic head in half.
Place the halved squash, garlic, onion, and carrots on the baking sheet, cut side up, and drizzle everything with olive oil. Season lightly with salt and pepper. Then flip everything cut-side down.
Roast in the oven for about 45 minutes or until the squash is tender. Let everything sit on the counter for about 10 minutes, or until safe to handle.
Add a cup of the vegetable stock to your blender. Scoop out the flesh of the squash, peel the onion, squish the garlic out of its skins, and spoon up the roasted carrots and add to your blender. You can also place in a large pot and use an immersion blender or blend in batches.
Blend until the mixture is very smooth. Continue adding stock and blending until the soup is at the consistency you want. At the very end, blend in the sour cream.
Divide into bowls and served garnished with sour cream, a touch of fresh cracked pepper if you have it, and the fresh parsley.