Heat the oil in a large pan, then add the mustard seeds, after a couple of minutes they will start to pop. Don't let them heat for any longer or they will burn.
Add the onions leeks and season them.
Saute them for about 5 minutes until they have started to soften.
Add the carrots and allow them to cook gently for 5 minutes.
Add the stock and bring to the boil. When it has reached boiling point, reduce it to a simmer and leave it to cook away for about 20 minutes until the carrots have softened.
Turn of the heat, add the milk and whizz with a hand blender or in a food processor until smooth.
Serve with a swirl of milk or a dollop of yoghurt and some chives.