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In a large pan, heat the oil with ½ tsp salt and some pepper. Add the leeks and potatoes, cook for 5 mins over a medium heat, stirring constantly, then add the garlic and squash. Continue to cook for 10 mins to give everything a bit of colour.
Add the coconut milk and 1 litre water, or enough to cover the mixture. Place the lemongrass on a board and crush with the flat end of a cleaver or knife, then add to the soup, turn the heat up and bring to the boil, then reduce the heat to a simmer. Cook for 20 mins until the squash is mushy. Remove the lemongrass.
Blend until smooth (the soup should be nice and thick, but add a splash of water if it has become too thick). Return to the saucepan and put back onto the heat. Taste and season again, then serve with a bowl of snipped chives and one of Chilli shrimps (see below), allowing guests to help themselves.