Place a medium saucepan over medium heat and add the oil. Bring to temperature. Add the mustard seeds, spices and curry leaves to the hot oil and allow to temper for 30 seconds. Add the onion and garlic and fry for two minutes. Do not allow the garlic to burn.
Cook for a further minute before adding the chopped tomatoes. If using fresh tomatoes, add a splash of water here too. Cover the pan with a tight fitting lid and leave to simmer for 5 minutes.
Add the lentils, water and salt. Bring to a boil then reduce to a simmer. Cover with the lid again and allow to simmer for 10 minutes, stirring occasionally.
Remove the lid and test the lentils - they should be just slightly undercooked and still just about holding their shape. Stir in the kale and another splash of water if the lentils are looking too thick. Allow to simmer for a further 5 minutes.
Remove from the heat and top with the coriander, vegan yogurt and chilli flakes. Right before serving, squeeze the lime over the top. Serve with rice, roti or just on its own.