Before cooking, follow on-pack instructions to rinse and pick over uncooked lentils.
Add the uncooked lentils, onion, turmeric, chilli and tomatoes to a saucepan. Add enough water to just cover the uncooked 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 the cooked lentil mix, stir and season if required.