Add ½ cup of white poppy seeds to a grinder and grind to make a fine powder. Measure out 4 tablespoons of powder and freeze the remaining for later use.
Heat mustard oil in a pan over medium-high heat.
Once the oil is hot, add cumin seeds, kalonji seeds, and dry red chilies and let them crackle for 3-4 seconds.
Drain the water from the cubed potatoes and add them to the pan. Fry for 2-3 minutes.
Now add 4 tablespoons of poppy seeds powder, salt, and green chilies to the pan along with ½ cup of water and mix well to coat the potatoes with the mixture. Reduce the heat to low.
Cover the pan with a lid and cook until the potatoes are tender, stirring a few times in between.
Remove the lid and cook uncovered until the sabji is almost dry. Check for salt and add more if required. Serve hot!