Combine the lentils, 2 boxes of chicken stock and turmeric in a heavy pot and bring to a boil. Cover, leaving the lid slightly ajar, turn heat to low and simmer for 30 minutes.
While soup simmers, peel potatoes and cut into 1/2-inch cubes. When soup has cooked for 30 minutes, add cut potato. Cover, leaving the lid slightly ajar again, and continue simmering for another 30 minutes.
During the second simmering period, put the garlic and ginger into an electric blender. Add 4 1/2 T water and blend until you have a smooth paste. (I used a food processor for this step. You don't need it to be a paste but it should be finely chopped.).
Once the soup base has finished cooking it needs to be pureed. Blender 3 batches. (I left one batch unblended to give the soup more texture.) Put pureed soup in a large bowl and set aside. Rinse and wipe out your soup pot.
Place cooked and cubed chicken in a separate bowl and sprinkle with salt and pepper. Toss to mix. Next, pour oil into soup pot and set over medium heat. When oil is hot, put in the garlic/ginger paste, cumin, coriander and cayenne. Fry, stirring continuously, until the spice mixture is slightly browned and separates from the oil. Put in chicken pieces and fry for 1-2 minutes. Add 14 1/2 oz. chicken stock and bring to boil. Cover and turn heat to low; simmer for 5 minutes.
Pour in pureed soup and lemon juice. Bring to a simmer and serve.