Peel the squash, remove the seeds, then cut the flesh into bite-sized cubes. Slice the pepper into thin strips. Put aside the prepared vegetables.
Deseed & finely chop the chillis, then soak in warm water for 10 minutes.
Chop the spring onions. Remove the hard skin of the lemon grass, then finely slice the white part.
Blend the spring onions, lemon grass, garlic and chillie in a small food processor until you get a rough paste.
Heat the oil in a large pan, add the paste and stir for a minute. Add the powdered spices (ground coriander, turmeric, paprika) and stir for another minute. Pour in the coconut milk, stir for 5 minutes, then add the vegetable stock. Bring to the boil.
Add the squash and pepper pieces, cardamom seeds and cinnamon stick. Simmer for 20-25 minutes, under cover on a low heat, until tender.
Add the soy sauce, sugar and lemon juice and cook for 2 more minutes.
Just before serving, sprinkle with chopped coriander leaves.