Finely dice the onion and celery. Heat some olive oil in a pan, and cook the onion and celery until they start to brown. Add the butternut squash and garlic and cook for a couple of minutes before adding the stock. Stir, bring to the boil, then reduce the heat and simmer for 30mins. Add the turmeric, ginger, bay leaf and rosemary sprigs whilst the soup is simmering. Remove the bay leaf and rosemary sprigs, then blend to a smooth consistency with a hand blender. Add lemon juice and season to taste.