Rinse the lentils thoroughly then put them in a pan water. When the water is boiling, remove the foam, then add the turmeric and ginger pieces and cook at a low temperature, under cover for about 45-60 minutes, stirring occasionally.
When the lentils are soft and creamy, turn off the heat, remove the ginger pieces and season with salt.
Make the tadka (tempering): in small pan fry the onions and garlic in the oil until soft, then add the spices (cumin, coriander, chilli) and fry for 1-2 minutes on low temperature.
Stir the tadka into the dhal. If it's not thick enough, cook for a further few minutes to your liking.
Serve with fresh, chopped coriander scattered on top.