Whisk curd with besan and water and keep aside. Heat oil and add mustard seeds. Once they crackle add the curry leaves. Add onions, ginger and green chilies and sautethem. Once they sweat add the tomatoes. Toss them and add turmeric.
Quickly stir it and add the yogurt. Season with salt and put back on flame and cook till it thickens. Serve hot garnished with fresh coriander leaves.