First grind tomatoes and garlic. Just churn for 5 secs..not to make a paste. keep aside.
Make two perpendicular deep cuts in baingan just as much to make a flower in 4 piece.
Dice potatoes into medium size.
In a pan, add oil, then cumin & mustard seeds, add finely chopped onion, saute for 2 mins, add all masalas. then add tomato and garlic mix. and saute well till it leave oil.
Add baingan and potatoes and green chillies and salt.
Put the heat off after 1 whistle and after it opens, add sugar and mix.