Thinly slice the onion, garlic and bell pepper. Peel and dice the sweet potatoes.
Heat the oil in a saucepan and gently sauté the onion for 5 minute, then add the garlic, sliced pepper and sweet potatoes. Sauté for another 5 minutes.
Add the spices, stir for a minute until fragrant, then add the tomato purée and chopped tomatoes. Stir for a minute or so, then stir in the lentils.
Pour in the stock and bring to the boil. Gently simmer for about 25-30 minutes until the vegetables are soft.
Using a potato masher, crush the sweet potatoes until you get a thick, textured, creamy stew. Stir through the chopped coriander leaves and some lime juice to your taste.
Serve with basmati rice.