1. Rinse soup bones and place in a pot with the water. Bring to a slow simmer, skimming when necessary.
2. When well skimmed and boiling, add washed lentils. Return to the boil, reduce heat to low, cover and simmer gently for 1 hour. Do not stir.
3. Meanwhile in a separate pan fry onion in oil until soft and lightly browned, add garlic cook for 1 minute more. Stir in tomatoes, coriander and salt and pepper to taste.
4. Remove bones from pot and discard. Add onion mixture to lentls, cover and simmer gently for further hour or until thick. Take care that soup does not catch to base of pot.
5. Serve hot in small bowls with flat bread and salad.