Heat the oil in a large pan over a medium heat and fry the onions and ginger for 10 mins, stirring occasionally until softened and starting to colour. Stir in the garlic, curry powder, ground coriander and dried chillies, if using, and cook for 1 min more.

Tip the mixture into a large slow cooker along with all the remaining ingredients, except the fresh coriander. Add 2 litres water. Cook for 8 hrs on high, or overnight for 15 hrs on low. Stir well, then blitz using a hand blender until smooth. Ladle into bowls and scatter over the fresh coriander to serve. Once completely cool, the soup will keep chilled in an airtight container for 48 hrs or frozen for up to two months. Reheat in a pan over a low heat or in the microwave until piping hot.