Melt the butter in a saucepan and sweat the onion, garlic and carrots, with a pinch of salt, for 5-6 minutes, or until softened.
Meanwhile toast the cumin seeds in a dry frying pan until fragrant. Add the toasted cumin seeds and Madras curry powder to the vegetables and cook a further 2-3 minutes.
Add the stock, bouquet garni and 500ml/18fl oz water and bring to the boil. Reduce to a simmer and cook for 30 minutes.
Remove the bouquet garni, then blend the soup in a blender until smooth. Pass through a sieve into a clean saucepan and season to taste with salt and freshly ground black pepper.
To serve, ladle the soup in serving bowls and sprinkle with chopped fresh coriander.