Fry chopped onions in the olive oil until starting to brown. Add garlic, then cumin and coriander. Cook for a couple of minutes, then add marrow. Cook for a further 5 minutes, then tip in lentils and mix thoroughly.
Add chopped tomatoes, stock and the finely chopped watercress. Bring to the boil then reduce and simmer for half an hour, or until lentils are soft and mushy.
Coarsely blend the soup with a hand lender. Season to taste. Allow to cool before serving.