Heat the oil in a large pan and add the onions, chilli, garlic and lemon grass. Cook for 5 minutes or until the onions have softened but not browned, stirring occasionally.
Add the lentils and spices. Pour in the coconut milk and 1 ½ pints/900ml water and stir. Bring to the boil, reduce the heat and simmer for about 40 minutes, until the lentils are soft.
Pour in the lime juice and add the spring onions and coriander, reserving a little of each for the garnish. At this point, the soup can be eaten as is or liquidised if you prefer a smooth soup. Season to taste, ladle into bowls, garnish and enjoy.