Wash the bitter gourd thoroughly. Using a serrated knife scrape off the ridges of the bitter gourd and collect them in a bowl. Do not throw away the scraping.
Cut the bitter gourd lengthwise on one side leaving the ends intact for easy opening to fill them.
Liberally rub the bitter gourd inside and outside with salt and set out in sun for couple of hours.
In the meanwhile, peel and grate the raw mango.
Wash the salted bitter gourd thoroughly and then squeeze it to remove excess water. Also squeeze the bitter gourd scraping to remove the bitter juices. You can collect the bitter juice and use it for drinking or other purposes.
Now mix the scraping, spices and raw mango together in a large bowl.
Using your thumb and forefinger, open the split bitter gourd and generously fill it with the prepared filling. Once done, tie it with a thread/ string to hold it in place for easy roasting.
Heat a flat bottom wide skillet. Add mustard oil. Now gently place prepared bitter gourds side by side, leaving enough space to turn them around.
Roast on slow flame till done. You can cover the lid for few minutes for faster cooking.
Serve as a side dish or pickle as per your preference.