Rinse the green lentils under cold running water until the water runs clear. You do not need to soak them.
Place the rinsed lentils, 4 cups of water, salt, and turmeric powder in a pressure cooker or a heavy-bottomed pot.
If using a pressure cooker, cook on medium heat until you get 4 to 5 whistles. Let the pressure release naturally. If using a pot, bring to a boil, then reduce heat, cover, and simmer for 40 to 50 minutes, or until the lentils are soft but still hold their shape.
While the lentils cook, prepare the tempering (tadka). Heat the oil or ghee in a small pan over medium heat.
Add cumin seeds and mustard seeds. Let them splutter.