First make the soup. Heat the olive oil in a large pan over a medium heat. Add the celery and sauté for 2-3 minutes until tender. Stir in the potatoes and sauté for another 2-3 minutes. Pour in the stock and season with a pinch of salt and plenty of pepper. Bring to a simmer and cook for 10 minutes until the potatoes are just tender.
Add the courgettes to the stock mixture and simmer for another 2 minutes, then stir in the spinach until just wilted. Add the nutmeg if using and blitz with a hand blender until smooth, adding a little water if you thick the soup is a little thick. Reheat if necessary and then ladle into warmed bowls. Add a swirl of the crème fraiche to each one and tear the mint leaves on top. Give a good grinding of black pepper to serve.