Heat your oven to 200°C/Fan 180°C/Gas 6. Pierce your sweet potatoes all over with the tip of a knife. Pop into the oven. Roast for 30-45 mins till fully tender.
Halve the roasted sweet potatoes and scoop out the flesh. Pop into a blender with the tahini. Peel, core and roughly chop the apples. Add them to the blender.
Crush the seeds from the cardamom pods and add them to the blender along with 400 ml cold water.
Blitz till smooth, adding more water (or ice) if needed.