Preheat the oven to 400°F (200°C) for a regular oven or 380°F (190°C) for a fan oven.
Prep the Vegetables: In a large pan, combine butternut squash, carrots, onions, garlic, thyme sprigs, and parsley. Season with salt and pepper. Drizzle with olive oil and toss to coat.
Roast the Vegetables: Roast in the oven for 45 minutes, stirring every 15 minutes, until tender and slightly browned. Remove thyme sprigs.
Transfer the roasted vegetables to a large pot and add the prepared stock.
Purée: Use an immersion blender to purée the soup until smooth.
Alternatively, purée the soup in batches using a stand blender. If using a stand blender, allow the soup to cool slightly before blending in batches to avoid hot splashes then pour the soup back into the pot.)
Add chili powder and dried herbs.
Stir in heavy cream if using
Place the pot over medium-low heat and cook for about 10 minutes until warmed through.