Quarter the apples and slice out the cores and seeds. Roughly chop the apples and pop them in a blender. Trim the dry ends off the celery stick, then roughly chop it and add to the apples. Tuck in the baby spinach leaves and spoon in the tahini.
Add a few ice cubes to the blender and pour in 250ml cold water. Blend for a few mins till the smoothie is combined. Serve straight away.