1. Take the pesto out of the freezer and allow to defrost for about 10 minutes. Meanwhile, heat the oil in a large pan over a medium-high heat and cook the onion, garlic and leek for 5 minutes. Add the butternut squash and cook for a minute more, then pour in the stock and bring to the boil. Turn down to a simmer, cover with a lid and cook for 15–20 minutes until the squash is soft.
2. Remove from the heat and blitz in a food processor or with a stick blender until smooth. Stir in the crème fraîche, season, then return to the pan and bring back to the boil. Spoon into bowls and stir some of the pesto into each serving.