Halve your onion squash and scoop out the seeds. Chop it into small chunks. Slide into the oven and roast for 30 mins. The squash should be soft. Leave to cool.
Peel, core and chop the pears and apples. Crush the cardamom in a pestle and mortar. Discard the papery husks. Finely grind the cardamom seeds. Peel and grate the ginger.
Place the squash (skin on), apples, pears, cardamom and ginger into a blender. Add 500ml water and blend till smooth. Add more water as necessary.