Spray the bottom of a large pan with the low fat cooking spray, heat the pan and then when the oil starts to bubble add the onion and sauté for about 3 – 4 minutes, continually stirring until softened.
Once the onion is softened stir in the cumin seeds, mustard seeds and curry powder for about 30 seconds.
Then stir in the lentils, stock and tomatoes. Bring to the oil, then turn down to simmer for about 20 minutes until the lentils have softened.
Blitz the mixture until smooth in a food processor or use a stick blender. Return to the pan, heat, bring to the boil then turn down to simmer and add the sweet potato chunks. Put the lid on and cook until softened for about 20 minutes, stirring occasionally.
Add the chickpeas and heat through for about 3 minutes.
Season to taste and serve in bowls with the coriander scattered over.