Preheat the oven to about 210ºC.
Slice the butternut in half (lengthways), and scoop out the seeds.
Slice the onion in half, remove the outer skin, top and tail it, and slice each half in half (so it's quartered).
Dice the carrots and peel the garlic cloves.
Place the butternut, onion, carrots and garlic on a large oven tray, and drizzle the olive oil all over.
Place in the oven and roast for about 45 minutes, until the butternut flesh is soft.
Remove from the oven and allow to cool until you can handle the butternut, about 10 minutes.
Scoop the butternut flesh out of its skin, into the pot, along with the onion, carrots and garlic.
Add the vegetable stock, and bring to a simmer.
Turn off the heat, and blend with an immersion blender, and season to taste with salt and pepper.
Serve with a garnish of fresh coriander, chilli flakes or coconut cream (all optional).