Blitz the soup with a handful of mint leaves for an extra-fresh flavour.
Place all of the ingredients, except the yogurt and ice, in a liquidiser or blender. Don’t overfill your machine – you may need to do this in two batches. Whizz everything for a couple of minutes until smooth and speckled with the watercress.
Season if you want to, then serve straight away or chill until needed. The soup will keep in the fridge in an airtight container for up to a 2 days (give it a good stir before serving) or can be frozen for up to 1 month. Serve the soup drizzled with yogurt and an ice cube or two to make it even more refreshing.