Wash the lentils in several changes of water. Cover the lentils with water to about 1cm above the level of the lentils and add the turmeric. Bring to the boil and simmer for 30 minutes or so until the lentils are cooked. Stir in the salt, cover and leave until needed.
Just before serving heat the butter in a frying pan. Add the garlic and gently fry for a couple of minutes until soft. Stir in the cumin.
Add some of the lentils to the frying pan and stir well so that they are well combined. Pour the frying pan mixture back into the sauce pan with the other lentils and heat through gently over a low flame.