Scrape the bitter gourd/karelas. Keep the scrapings. Slit the karelas. Remove the seeds and discard them. Apply salt and leave it for 1/2 hour. Wash them thoroughly. Add salt to the scrapings and leave it for 1/2 hour. Wash in running water to remove excess salt and bitter taste. Heat oil in a pan. Add chopped onions and cook for 2-3 minutes. Add ginger garlic paste and half of the scrapings. Cook for 4 minutes. Add half the turmeric powder, half the coriander powder and half the dry mango powder/amchur. Cook for another 10 minutes. Stuff the karelas with this stuffing and deep fry in medium hot oil until dark brown. Heat oil, add sliced onions and cook till light golden brown. Add rest of the scrapings and cook for 7-8 minutes. Add remaining turmeric powder, coriander powder, salt and dry mango powder/amchur. Mix well and add stuffed karelas. Cook covered for 2-3 minutes. Serve hot.