Bruise the cardamom pods and shake out the seeds, then roughly crush them with a pestle and mortar.
Peel and chop the onion and sweet potato, peel and finely grate the ginger, then peel and crush the garlic.
Heat the oil in a large pan over a low heat. Add the onion and a small pinch of sea salt and cook for 10 minutes, or until soft and sweet, stirring often.
Stir in the sweet potato, ginger, garlic and crushed cardamom seeds. Cook for 2 minutes, before adding the coconut milk.
Let it simmer for 1 to 2 minutes, then stir in the stock. Cover with a lid, and leave to simmer gently for 15 minutes.
Liquidise the soup with a stick blender until smooth, then season to taste with a pinch of salt and black pepper and a squeeze of lemon juice.
Ladle the soup into bowls, then serve with broken up pieces of poppadoms on the side, if you like.