Peel and chop the onion. Peel the garlic clove and crush or finely grate. Separate the coriander stalks and leaves (set the leaves aside for later), and finely chop the stalks.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the onion, garlic and coriander stalks. Sprinkle in a generous pinch of salt and pepper and fry for 5 mins, stirring occasionally, till slightly softened.
While the vegetables are cooking in the pan, prepare your butternut squash. Halve the squash, scoop out the seeds, then peel it. Coarsely grate the flesh.
When the vegetables have softened, slide in the grated squash and top up with 750ml boiling water. Crumble in the stock cube and stir well to combine. Bring to the boil, then reduce to a simmer and cook for 20 mins.
Pop a small frying pan onto a medium-high heat. Scatter the cashew nuts into the dry pan and cook for 3-4 mins, tossing frequently, till golden and toasted on all sides. Tip into a food processor. Add the coriander leaves and the zest and juice of the lemon. Sprinkle in a little salt and pepper and pour in 2 tbsp oil. Blitz until well combined into a loose pesto. Scoop into a bowl and set aside.
When the soup is cooked, use a stick blender to whizz till smooth. No stick blender? Simply pour the soup into a food processor (in batches if necessary) and blend.
Ladle the soup into deep bowls and top with spoonfuls of the cashew pesto.