Heat the oil in a large saucepan over a medium heat. Add the cumin seeds and fry for a minute or two until starting to brown.
Stir in the onion, reduce the heat to low and cover. Cook, stirring often for 10 minutes or until soft.
Add the garlic, ginger, turmeric, ground coriander and chilli (if using), stir well and cook for a further minute.
Stir in the sweet potatoes and mix well, ensuring that the spicy onion mix covers them well.
Add the rest of the ingredients, bring to the boil then reduce to a low heat, cover and simmer for 20-30 minutes until the lentils and sweet potatoes are cooked through.
Stir occasionally to prevent sticking and add a dash more water if it's starting to look dry.
Serve with the finely chopped coriander leaves and rice or chapatis.