Peel the butternut squash, then halve it and scoop out the seeds. Use a box grater to coarsely grate it. If you have a food processor you can whizz the squash through the grating attachment to make this extra quick. Peel the onion and finely chop it. Peel and grate the garlic and ginger. Separate the coriander leaves and stalks (pop the leaves aside for later in the recipe), and finely chop the stalks.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the onion and sprinkle in a pinch of salt and pepper. Fry for 3-4 mins, stirring occasionally, till starting to soften. Stir in the chopped garlic, ginger, coriander stalks and 1 tbsp Thai green curry paste and cook for another 1 min.
Tip the grated squash into the pan and crumble in the stock cube. Pour in 750ml boiling water and stir well. Pop on a lid. Bring back to the boil, then simmer for 15-20 mins, till the squash is tender.
Use a stick blender to whizz the soup till smooth. No stick blender? Simply transfer the soup into a blender (in batches if necessary) and blitz.
Ladle the soup into bowls and serve garnished with the reserved coriander leaves.