Wash the daal (lentils) thoroughly. Place in a heavy bottomed pan with the water. Cook partially covered over medium heat until it reaches a boil. Stir frequently and skim off any froth that rises to the top.
Add the salt, turmeric powder, ginger/garlic mix, and green chili. Cook for 15 to 20 minutes until the daal becomes soft and has a creamy consistency. Add water as necessary.
Heat the cooking oil in a fry pan until it is quite hot.
Add the cumin and mustard seeds to the oil. Fry briefly on high.
Add the curry leaves and onion, cook a few more minutes.
Add the tomato, chili powder and 1/2 the coriander leaves. Fry a few more minutes.
Remove this onion-tomato mixture (the tardka) from the heat and pour over the daal. Sprinkle with the remaining coriander leaves.