Preheat the oven to 200°C/400°F/gas mark 6. Put the butternut squash in a roasting dish, drizzle with oil and season with salt and pepper. Roast in the oven for around 20-30 minutes until soft.
Heat the oil in a medium sized saucepan on a medium low heat. Add the onion and cook for 5 minutes or until the onion is translucent. Add the garlic and cook for a further minute. Add the rosemary and continue to cook for a couple of minutes.
When the squash is cooked, add it to the pan along with the tomato purée and stir. Add the stock and bring to the boil, then lower the heat to a simmer. Half cover with a lid, and cook for 15 minutes, stirring occasionally. When the squash pieces begin to disintegrate, remove from the hob and allow to sit for 5-10 minutes.
Using a hand blender, purée the soup in the pan (alternatively, allow the soup to cool and transfer to a blender to purée). Return the soup to the hob to warm through and, if desired, add the cream.
Serve with parsley and a drizzle of cream to garnish.