Preheat your oven to 170°C. Slice the butternut squash in half, scoop out and discard the seeds and put both halves on a baking tray.
Drizzle with a little olive oil and lay the rosemary sprigs in the hollow of each half of squash. Bake for 45 minutes.
Allow the squash to cool slightly, discard the woody sprigs but keep the leaves from the rosemary, then peel off the outer skin of the squash and chop the flesh into cubes.
In a large saucepan sauté the onions, celery and carrot in the oil for about 5 minutes. Mix in the
squash pieces and the pinch of chilli flakes, if using, and then pour in the vegetable stock. Allow this to simmer gently for 15 minutes.
Season with freshly ground black pepper and a little sea salt to taste. When the mixture has cooled slightly, ladle it into a
blender (or use a handheld blender).
Mix in the crème fraîche or soured cream and whizz until smooth, about 15 seconds.
Pour the soup back into the saucepan, heat through and serve topped with a dollop of crème fraîche.
Reprinted with permission of Mary McCartney