Peel and finely chop the onion, garlic, chilli, turmeric and ginger.
Peel and dice the sweet potatoes and carrot. Chop some tomatoes - enough to have half a cup.
Cut the red pepper into chunks.
Heat the oil in frying pan and gently sauté the onion for 5 minutes, then add the garlic, turmeric, chilli and garlic. Stir for a minute, then add the mustard seeds, cumin seeds and coriander seeds. Stir for another minute.
Add the chopped tomatoes, red pepper, garam masala, then season with salt & pepper.
Rinse the lentils, then add them to the pot together with the sweet potatoes and carrot and pour over about a litre of water.
Gently simmer for 20 minutes under cover, until you get a mushy stew. You can mash the sweet potatoes further with a fork for a smoother texture.
Stir in the lemon juice and the chopped coriander leaves.
Serve warm.