Heat mustard oil in a non stick pan till it smokes. Switch off heat.
Cut potatoes into ½ inch cubes. Drain and grind khus khus to a fine paste.
Heat the mustard oil again, add onion seeds and sauté till fragrant. Add potatoes and mix well. Cover and cook for 5 minutes.
Add khus khus paste and mix. Cover and cook again till the potatoes are done.
Slit green chillies. Add salt, sugar and green chillies and mix. Add ½ cup water and mix. Simmer for a minute.
Add ghee and serve hot.