Grind the chillies, spring onions, garlic and lemongrass to a paste in a processor, adding a little oil if necessary. Heat 1½ tbsp oil in a heavy saucepan over a medium-low heat. Add the chilli paste and cook, stirring until it is fragrant and no longer tastes raw.
Add the coriander, turmeric, and paprika and stir for 1 minute. Add the coconut milk and simmer, stirring for 5 minutes.
Stir in the stock and bring to a simmer.
Reduce the heat to low and add the butternut squash, red pepper, cardamom, cinnamon stick and salt to taste. Simmer, partially covered until the butternut squash is tender and the sauce is reduced, about 20-25 minutes.
Stir in the soy sauce, sugar and lemon juice and simmer for another 2 minutes. Serve garnished with coriander leaves.
This is quite a wet stew. It can be thickened with cornflour paste if required. Serve with plain rice.