Wash the eggplant. Cut it in 2 to 2.5 inch batons. Soak in water while you prepare other ingredients.
Peel the potatoes and cut them lengthwise. Cut them a little smaller than eggplant.Soak in water while you prepare other ingredients.
Peel and grate the onion, you should have about 1/2 cup of grated onion. Chop the tomatoes. Pound the ginger and garlic into a thick paste using your mortar and pestle.
In a heavy bottom pot, warm up the mustard oil on medium low heat till its a bit smoky. Crackle the cumin seeds and hing for 10 seconds in hot oil.
Add the onions and brown the onions in oil for about 5 minutes until they are light golden. Do not brown too dark else the color and taste of curry changes.
Add the ginger garlic paste and saute for 15 seconds. Next add all the powdered spices - corinader, turmeric,chili powders. Add a tablespoon or two of water and brown the spices for 30 seconds or so.
Next add the tomatoes, sugar and salt. Mix and cook the tomatoes for about 2-3 minutes on medium low heat till they start to soften. Add the drained potatoes now, mix and cook the masala for another 4-5 minutes until you see oil seperating on sides.
Drain as much water as possibe and add the eggplant to the pot next. Mix around so that eggplant batons are coated well in the masala. Keep stirring around and cook the eggplant with masala for about 2 minutes.
Next add about 1.5 cup hot water. Mix, taste and adjust the salt. Bring to a slow boil and cover the pot. Let cook for about 8-10 minutes on medium low heat until the potatoes are fork tender and eggplant is cooked through.
Once the vegetables are cooked, switch off the stove and finish with the amchoor, garam masala and kasuri methi while still warm. Mix very gently. Cover and let the curry rest for atleast 20 minutes.
Garnish with chopped cilantro. Serve with rotis or rice.