Heat the oil and butter in a large pan over a medium heat.
Add the onion, leek and garlic and soften for 5 minutes, stirring occasionally.
Add the butternut squash and fry for another 5 minutes (or until onions and leeks softened), stirring again.
Add the stock and pepper and bring to the boil.
Lower the heat and simmer for 15 minutes under a lid.
Add the spinach leaves, replace the lid and allow to wilt (about 5 minutes).
Blend the soup and serve garnished with onion salad crispies.