Chop all the vegetables. Heat three tablespoons oil in a kadai, add the cumin seeds and hing powder. When the cumin seeds splutter add the onions,capsicum, green chillies and ginger into the hot oil and fry till the onion turns golden. Add tomatoes and fry till it becomes mushy. Add cauliflower, potatoes and turmeric powder, fry a bit, add salt, sugar and quarter cup of water. Cover and cook till the vegetables are cooked and the curry is dry. Add the rasam powder, coriander leaves and the remaining one tsp of oil-fry till you get a lovelyaroma. Remove and serve piping hot with rotis or rice.
Note: Plain potato curry can also be made the same way.