Thoroughly wash the parwal. Using a sharp knife, cut off and discard the pointed ends. If the parwal are fresh and bright dark green, I don't peel off the skins. If I see that the skins are pale green or yellow and bruised, using a peeler, peel off the skin.
Lengthwise, cut the parwal in half. If you can cut the seeds easily using a knife, they are soft. If the seeds feel hard, scoop them out. Slice each half vertically into thin strips.
Peel, halve and cut potato in a similar fashion. Make sure to keep the size of parwal and potato similar. Keep potatoes soaked in water until ready to use.
In a cooking pan, heat up the mustard oil until slightly smoky on medium heat. Reduce the heat to low. Wait for a minute or so and temper the hot oil with cumin seeds. Let crackle. Also add the hing.
Next, on low heat (or take off the pan off stove if needed), add red chilli powder, turmeric and corinader powder. Fry the powdered spices in oil for 20-30 seconds taking care not to burn. If needed add a splash of water.
Now, introduce the potatoes. Sprinkle ¼ teaspoon salt. We will first cook the poatoes for 3-4 minutes to give them a head start since they take slightly longer than parwal to cook. Fry the poatoes with spices stirring them often.
Ater 3-4 minutes, add the sliced parwal. Sprinkle black pepper powder and salt. Stir to coat the parwal in spices. Fry without covering for 2 minutes or so. You will see that the color of parwal is getting deep green. Add a splash of water (2-4 tablespoon) and then cover the pan. Tip - Keep in mind that parwal wont release a lot of water, so check 1-2 times in between to make sure that parwal are not sticking. Let the sabji for 8-12 minutes or until both parwal and aloo are tender. I don't prefer mushy or overly soft vegetables so please adjust the coking times as needed.
Finish the sabji with garam masala and amchur powder. Bump up the heat and bhuno (fry) for 2 minutes on high heat. Garnish with chopped cilantro leaves. Serve!