Remove the tender leaves from the stems, finely chop the leaves. Keep aside.
If the stems are tender, cut into 1- 1.5 inch long pieces, wash thoroughly and keep aside. If not tender, I would recommend avoiding using the stems.
Boil the stems in salt water till tender. Drain from the water. Keep aside.
Heat oil in a Kadhai, add the potato and potol, sprinkle salt, shallow fry, some 8-10 minutes.
Add the ginger and coriander paste, cook over a medium flame for 5 odd minutes till oil starts to release from the masala.
Turn the heat to low, gently pour in the milk, while stirring continuously, cook for another odd 5-7 minutes.
Throw in the bottlegourd stems and leaves, continue to cook till the vegetables are tender. Sprinkle in the sugar.
In a separate pan, heat the ghee, throw in the kalonji and green chilies. Allow them to splutter.
Add the tempered ghee over the curry, give it a hearty mix.
Adjust seasonings. Serve hot.