Quarter the apples, then slice out the cores. Coarsely grate them, with the flesh facing the grater so you are left with the skins. Keep the skins to go in the blender. Trim, peel and coarsely grate the beetroot. Peel and grate the ginger. Pop them all in your blender.
Trim the dry ends off the celery and roughly chop it. Roughly chop the watercress. Add them both to the blender. Pour in 250ml cold water and add a few ice cubes. Blitz till combined. Serve right away.