Trim the dry ends off the rhubarb sticks and roughly chop the rhubarb. Pop in your blender with 250ml cold water. Blitz till smooth, then pour the rhubarb mix into a sieve set over a bowl or jug. Use a wooden spoon to press down on the rhubarb fibres to squeeze out as much liquid as possible, leaving the dry fibrous strands in the sieve. Discard them.
Pour the rhubarb liquid back into the blender. Peel the oranges and break the segments into the blender. Peel the banana and break that into the blender too. Tuck in the baby leaf spinach leaves. Add a few ice cubes and blitz till smooth, trickling in a little more cold water if you think it needs it.
The rough with the smooth
Blitzing the rhubarb with water and the squeezing the liquid through a sieve means you get a delicate rhubarb flavour in your smoothie without the tough, chewy strands. If you don't want to sieve the rhubarb, you can just leave it in whole (you will get a very thick smoothie) or you can poach the rhubarb with the water for 5-6 mins till soft, then cool and make the smoothie.