Add coriander leaves and salt, and let the mixture simmer for at least ten minutes on low heat.
Turn off heat and add lemon juice.
When the mixture cools down , put it in a mixer and blend it until smooth.
Heat the soup before serving it.
Notes
If you are making this soup for babies, use powdered cumin seeds (jeera powder) instead of whole seeds. Because the seeds may not get ground properly when you blend them along with sweet potato and lentils.