Heat the oil in a medium saucepan on a medium flame, then fry the onion for eight minutes, until soft and caramelised. Add the chorizo and cook for 2 minutes until it browns a little. Add the curry powder, chilli flakes, garlic and ginger, and fry for two minutes more, stirring continuously. Add the lentils, stirring through, then add the tomatoes, coriander stalks, 300ml cold water, a teaspoon of salt and a grind of pepper, and leave to heat through. Pour the coconut milk into a bowl and gently whisk until smooth and creamy. Set aside two tablespoons – for serving – then tip the remaining coconut milk into the soup pot. Bring the mixture up to a boil, turn down the heat to medium and leave to simmer gently for 25 minutes, until the lentils are soft but still hold their shape. Divide the soup between four warmed bowls, drizzle over the remaining coconut milk, scatter the coriander leaves on top.