Melt the butter in a large pan. Add the celery and ginger, then cook gently for 4-5 minutes until softened but not coloured, stirring occasionally. Stir in the carrot, cumin, maple syrup and lemon juice and season with a pinch of salt and plenty of pepper.
Pour the boiling water into the carrot mixture and bring to the boil. Reduce the heat and simmer for 45 minutes until slightly reduced and the carrots are tender.
Remove the soup from the heat and blitz with a hand blender until smooth. To serve, ladle into warmed bowls. Drizzle a little soured cream into each one and garnish down the middle with a thin line of coriander, if liked.