Melt the butter in a saucepan and sweat the onion, garlic, and carrots with a pinch of salt for 5-6 minutes until softened.
Toast the cumin seeds in a dry frying pan until fragrant.
Add the toasted cumin and Madras curry powder to the vegetables and cook for 2-3 minutes.
Add the stock, bouquet garni, and 500ml of water then bring to a boil.
Lower the heat to a simmer and cook for 30 minutes.
Remove the bouquet garni and blend the soup in a blender until smooth.
Pass the soup through a sieve into a clean saucepan and season with salt and pepper.
Ladle the soup into bowls and garnish with chopped fresh coriander.