Add the marinated karela peels and slices, and mix gently. At this stage, add the dry spices - turmeric powder, amchur powder, red chilli powder, and coriander powder. Mix well. Cover and cook for five minutes and then cook it without the lid on a very low heat for 15-20 minutes until the Karela Sabzi becomes crispy.
In a kadai or pan, heat 2 tablespoons oil. You can use any neutral flavored oil. Let the oil become hot. Then reduce the heat to a low and add the chopped karela. Mix chopped karela with oil and sauté on a low to medium-low flame. Stir often while sautéing. Sauté the karela for 4 to 5 minutes. Then add thinly sliced onions.
Squeeze all the water from the bitter gourd using your hands. Place the karela on a kitchen towel or muslin cloth to dry them completely. For making the onion and karela sabji, heat the oil in a deep non-stick kadhai. Add the onions and sauté on a medium flame for 1 to 2 minutes.
In the mixing bowl, combine the thin karela strips and the peels, add salt and give it a good mix and allow it to rest for about 20 minutes. Heat a heavy bottomed pan on medium flame, add mustard oil. Once the mustard oil begins to smoke, turn down the heat and add the fennel seeds, let these sizzle.