Heat oil in heavy pot on medium high heat. Saute onion and garlic until softened. Stir in lentils, ginger, cumin and coriander and coat with onion/garlic mixture. Add lemon slices, stock, tomatoes, cayenne, salt and pepper (you can add salt and pepper later, but like to put it in now).
Bring to boil, reduce heat to low, cover pot and simmer for about 45 minutes (it may only take 30 minutes) or until lentils are tender. If you like your soup thin, add more stock.
Take the pot off the burner, discard lemon slices, and puree soup in blender or food processor. Season if you have not already added the salt and pepper. Sprinkle with coriander (cilantro leaves). Alternatively, you can add the cilantro before you put the soup in the food processor.