Heat the oil in a medium saucepan. Dice the onion finely.
Cook the onion in the pan for about 10 minutes, until translucent. Salt generously. In the meantime, Chop the garlic and ginger finely.
Add the garlic and ginger to the onion, and cook for another minute.
Add the cumin, turmeric, coriander, cinnamon and chilli to the pan. Add the dried red lentils too, and stir well.
Add the tomatoes and the water to the pan. Stir again, then cover and adjust the heat to minimum. Cook for about 20 minutes, and check sometimes if more water is needed.
Serve with finely chopped fresh mint (and some naan bread if you fancy).