Preheat oven to 200°C/400°F/Gas Mark 6. Cut the butternut squash in half lengthwise, scoop out the seeds, and place cut-side down on a baking tray lined with parchment paper.

Add the carrots, onion, celery, and garlic to the tray and drizzle everything with olive oil. Season with salt and pepper and roast for 30 minutes.

Once the vegetables are roasted, let them cool slightly, then scoop the butternut squash flesh out of its skin and add it to a food processor with the roasted vegetables and 3 tablespoons of water. Blend until smooth.

Pour the mixture into a saucepan along with the vegetable stock. Bring to a boil and simmer for 15 minutes.

Serve hot with toasted sourdough bread.