Add the lentil, onion, turmeric, chilli and tomatoes to a saucepan and add enough water to just cover the lentils. Bring to the boil and simmer until lentils are soft, approximately 15-20 minutes. Add more water if required. Mash mixture and set aside.
In a frypan, heat oil and add seasoning ingredients. Fry until the flavour releases and mustard seeds start popping, approximately 3 minutes. Add to lentil mix, stir and season if required.