Heat the oil in a large pan and cook onions, stirring occasionally, until very brown. Stir in the garlic.
Add the lentils and stock and bring to the boil. Remove any scum. Stir in the cumin, coriander and chilli powder, if using, and simmer for an hour or until lentils have disintegrated, . Add salt if desired when lentils have begun to soften.
If necessary, add water to thin the soup to a light-cream consistency. Stir in the lemon juice and serve in warm bowls, garnishing each with a little parsley or dill.
*Variations: Add 125g(4oz) rice or broken vermicelli 15 minutes before the end. Or add 2 tomatoes, skinned and chopped, when the soup is ready and just let them soften. Or add 500g(1lb) well rinsed and shredded spinach and cook for a further 10 minutes.