Heat the olive oil and butter in a large pot over a low heat. Add the onion, leeks, celery, butternut and potatoes. Cover the pot and leave the vegetables to sweat for 10 minutes. Shake the pan regularly to prevent them from sticking to the bottom.
Increase the heat to medium. Add the salt, turmeric, ginger and cinnamon and stir for a minute or two. Add the stock and stir well. Leave to simmer until the vegetables are tender, about 20-25 minutes.
Set the pot aside to cool for a few minutes. Use a food processor or stick blender to puree the soup until smooth.
Return the soup to the pot and stir in the cream and sherry. Heat through over a low heat. Check the seasoning and serve.