Heat oil in a cooking vessel and add the mustard seeds and let them pop. Now add the red chillis and curry leaves and stir fry for a few seconds till the flavors are released in the oil.
Now add onions, ginger and garlic and saute till the onions turn light brown. Add the green chillis and all the pwds. Combine.
Add the tomatoes and stir fry till the oil seperates and it gets mushy.
Add the potatoes and combine and keep covered with a lid on low heat for 3-4 minutes.
Remove lid and cook further for another 3-4 mts stirring once in a while.
Add salt and enough water to cover the potatoes.
Bring to a boil and let it simmer on low heat till you get the desired curry consistency. Finally add the jaggery/sugar and combine. Turn off heat. Garnish with chopped coriander leaves.
Serve hot with rotis/chapatis/steamed white rice.