Heat the oil in a large saucepan. Add the onion with a pinch of salt, cover and fry gently for 8-10 mins, stirring regularly.
Once the onion is completely soft, add the ginger, garlic, lemongrass (if using) and chilli to the pan. Cook for 1-2 mins, until aromatic. Tip in the squash and nuts, pour over the stock and simmer for 15-20 mins, until the squash is tender to the tip of a knife.
Blend the soup until smooth and creamy. Taste and season with soy or fish sauce. Serve, adding any extras that you may have.
The soup will keep in the fridge for 3 days but also freezes really well.