Heat the olive oil in a heavy-bottomed soup pan over a medium-high heat. Add the onion, garlic, ginger, lemongrass, lime leaves and fresh coriander and sauté until the onions start to soften. Stir in the curry paste, then tip in the sweet potatoes and mix. Pour the coconut milk and stock into the pan and add the brown sugar, fish sauce and the lime zest. Squeeze the lime halves into the soup and then throw in the squeezed halves for good measure.
Bring to the boil and keep at a steady simmer until the sweet potatoes are cooked – roughly 30 to 40 minutes. Use tongs to carefully remove the lemongrass and lime halves and discard them, then blitz the soup until smooth. Add a bit of hot water if it’s too thick, then taste and season with more fish sauce and some black pepper if it needs it. Serve with chopped fresh coriander on top and a sprinkle of coconut – I am also a sucker for adding some chopped roasted peanuts and a squeeze of lime juice.