First, add the oil to a large frying pan and heat to a medium heat. Add in the mustard seeds. When they pop, add in the cumin seeds, coriander, turmeric and chilli flakes. Stir for a few seconds, then add the onion. Fry for approx 8-10 minutes on a low to medium heat until soft and browning.
Add in the garlic, ginger, and grated beetroot and stir for another few minutes.
Cook for a further 3-4 minutes, then add the lentils, water and coconut milk/yogurt.
Stir well, then turn down the heat and put the lid on the pan.
Simmer for 30 minutes, stirring occasionally. Add more water if needed.