In a large pan, heat oil over medium high heat. Splutter mustard seeds. Add onions, and sauté, stirring occasionally, until soft and translucent. Add curry leaves. Saute gain, stirring occasionally, until onions turn light brown.

Add ginger and garlic. Saute for a minute, or until fragrant.

Add a little water to cover the onions. Cover with a lid, and cook for one minute. Remove the lid. Once onion is cooked and the mixture is dry, add ground turmeric, ground coriander, and red chilly powder. Stir and sauté for a minute, or until fragrant.

Add tomatoes and enough salt. Add more water to make sauce. Cover, and bring to a boil. Reduce heat and simmer for 3-4 minutes, or until onions are cooked. Add sugar and continue to cook until heated through. Remove from heat.

Serve thattukada onion curry with thattukada chicken and parotta.