Peel and chop the onion, garlic, turmeric and ginger.
Peel and dice the sweet potatoes and carrot. Chop the tomato.
Heat the oil in frying pan and gently sauté the onion for 5 minutes, then add the garlic, turmeric and garlic. Stir for another 2 minutes. Add the mustard seeds, cumin seeds and coriander seeds, and stir for a minute. Add the tomatoes and season with salt & pepper.
Rinse the lentils, then add them to the pot.
Add sweet potatoes and carrots too, then pour over about a liter of water- just enough to cover everything well.
Add the bay leaf and cinnamon bark then bring to the boil.
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.