Halve the butternut squash, scoop out all the seeds and score both sides in a crisscross pattern with a sharp knife. Then season both halves with salt and pepper. Peel the sweet potatoes and chop them into fairly large chunks.
Put it all on a large roasting tray and spray with a little oil and roast for about an hour at 140°C. Don’t let it go too brown, but you want it to be quite soft.
When it’s cooked, let it cool down, then remove the skin from the butternut squash and place to one side.
In a large pan, soften the onion, carrots and celery, then add the chilli for a few minutes. If you keep the lid on the pan and the hob on low/medium, the veg will sweat itself into softening so you don’t need oil.
Once that’s all nice and soft, add the sweet potato, butternut squash and stock, then simmer it for about 10 minutes.
Next, add all your cooked ingredients to a blender and blend it into a liquid. Pour the soup back into your pan and place the pan back on the hob.
Consistency of soup is very much down to personal preference. If you like it thinner, add more water. If you want it thicker, keep it simmering for a bit longer. Once you’ve reached your desired consistency, take it off the heat and stir in the milk. Season it with salt and pepper to taste.