Gently cook the onion, celery and garlic in a large pan with the oil for about 4 minutes. When they have softened, but not coloured, and lost their raw aroma, add the ground coriander and cook for a couple more minutes - it will soak up the oil, so be careful not to burn the pan.
As soon as the coriander is cooked add the 1 litre of stock, 200 ml of water and the carrots. Bring to the boil, then cover and turn down low to simmer until the carrots are soft (about 15-20 minutes)
Remove from the heat and blend. The soup will cool a little during blending. Right before serving, blend in the fresh coriander. Some chopped coriander leaves can be sprinkled on top to garnish.
Notes
Have the stock prepared and ready to hand before adding the spices, as you'll need to add it promptly once they're cooked.Keeps well covered in the fridge for up to two days - but the fresh leaf coriander is best blended in to the soup right before serving to preserve its taste.