Thinly slice the onion. Heat oil in a pot and add the sliced onions to it. Make a paste of the Garlic,Ginger and Green Chillis and add this to the onions. Lightly grind Cumin and Coriander seeds and mix in onions and garlic mixture and cook on medium heat.
Add the Red Chillies, Salt, Curry leaves, and Tamarind Paste and fry for another 5-10 minutes.
Meanwhile, Mix 1 cup of Gram Flour in the plain yogurt and make into a smooth paste by adding small amouts of water at a time. Add this yogurt mixture in onions and mix and cook by constantly mixing. Keep mixing until it starts to come to a boil. Then turn heat level low and leave it to cook slowly until it thickens.