Place everything in a Blendtec or other high speed blender.
Puree until smooth and a completely uniform green colour appears (about 30-45 seconds). Thin with water or thicken with ice, if required, and blend again until smooth.
Serve in a chilled glass with a glass smoothie straw (if desired).
Recipe by Make Mine Gluten Free at https://www.makemineglutenfree.com/classic-green-smoothie/