Rinse the rice and lentils in cold water until the water runs clear. Set aside.
Heat the oil in a large pan with a lid. Add the cumin seeds and let them sizzle for 30 seconds.
Add the onion and cook for 4-5 minutes until golden.
Add the garlic, turmeric and chilli flakes. Stir and cook for 1 minute.
Add the chopped tomatoes, rice, lentils, salt and water. Stir well.
Bring to the boil, then reduce the heat to very low. Cover with a tight-fitting lid and cook for 20 minutes without lifting the lid.
Remove from the heat and let it sit covered for 5 minutes. Fluff with a fork and serve.