Heat 2 tablespoon oil in a pan and fry onions until light golden brown.
Remove from oil, let cool and keep aside. Grind to paste. This paste stays good in the refrigerator for 2-3 days and with addition of salt and turmeric, up to a week.
In the same pan roast long green chilies. Chilies should change color and have some blisters on it. Remove and keep aside.
Add remaining tablespoon of oil. Add ginger and fry until raw smell is gone.
Add ground onion paste and saute until oil separates.
Add ground powders, coriander powder, chili powder, salt, turmeric and let the powders roast in oil along with the onion paste.
Add water, tamarind paste and bring to a boil.
Let the salan simmer on medium flame for 4-5 minutes or until the oil begins to separate.
Add green chilies and continue to simmer for addition 5 minutes or until salan is done.
Garnish with cilantro and serve with rice or biryani.