To make potato sabji, boil potatoes, peel them and cut them into medium size pieces. Now in a pan heat oil and add cumin. Let it brown then add curry leaves and chopped green chillies. Mix everything well and add potatoes to it. Add rock salt, sugar, lemon juice, pepper powder and give it a good mix. Cook it on medium heat for 5 minutes. Add coriander leaves and turn off the heat.