Chop the tomatoes into bite sized pieces.   Chop the coriander stalks and reserve the leaves.

Put the olive oil in a saucepan over a medium heat.  Add the onion and fry until soft but not coloured, for around five minutes.  Add the garlic and cook for a further minute, add the harissa and cook for another couple of minutes, stirring.  

Add the lentils, stir them in and then add the tomatoes, coriander stalks, coconut milk and 500ml water, salt and pepper to taste

Bring the soup to the boil and then turn down to a simmer for 20 minutes.  Add a little more water if you feel the soup is too thick.

Ladle into bowls, sprinkle with the reserved coriander leaves and serve.