Heat your oven to 220°C/Fan 200°C/Gas 7. Slice the woody stem from the onion squash. Quarter the squash. Scoop out the seeds. Pop it onto a baking tray. Season well.
Slice the garlic bulb in half widthways. Add to the tray. Roast on the top shelf of the oven for 30 mins, or till everything is golden and tender.
Scoop the roasted squash from its skin. Carefully squeeze the roasted garlic cloves from their skins. Spoon both into a blender with the coconut milk (reserve 2-3 tbsp). Use a mortar and pestle to grind the cardamom pods. Discard the papery skins and add the ground seeds to the blender. Blitz till smooth.
Whizz in 250ml boiling water, little by little, till the soup is as thick or thin, rich or delicate as you like. Adjust the seasoning to taste.
Serve with a swirl of the reserved coconut milk, and a little extra dusting of ground cardamom seeds if you like.