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 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.