Pressure cook the toor dal with the haldi and water for 10 minutes. You want it cooked but not mushy.
Heat the oil in a saucepan, add the mustard seeds. When they splutter, add the cumin seeds if using and the hing powder. Stir, then quickly add the curry leaves. Stir again and add the green chillies.
Fry the green chillies and the curry leaves for a few seconds, then add the onions.
Once the onions are half-done, add the haldi, chilli powder, water, tamarind concentrate cherry tomatoes and salt. Bring to the boil and simmer until the onions are well-cooked.
Add the cooked dal and bring to the boil.
Turn off the heat and garnish with chopped coriander leaves .Serve as an accompaniment to plain rice.