Add the oil to a small frying pan with the onion. Fry on a low heat for approx 5 minutes, then add in the garlic.
Fry for a further 5 -7 minutes until soft and caramelised, then add in the spices. Fry a further 3 minutes.
Turn off the heat and season with a pinch of sea salt.
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 and ginger and stir for another few minutes, then the chopped tomatoes.
Cook for a further 3-4 minutes, then add the lentils, water and coconut milk.
Stir well, then turn down the heat and put the lid on the pan.
Simmer for 25 minutes stirring occasionally. Add more water if needed.
Season well with salt, pepper, garam masala.