Peel some of the bumpy skin off of the bitter melons, then make a slit along the middle and spoon out any seeds. Set the bitter melons aside for now.
Mix all of the filling ingredients together in a bowl.
Fill about 1 tablespoon of masala into each karela (add a bit extra to the larger karelas). Tie them with kitchen twine to hold the masala inside.
Pour 2 cups of water into the steel insert of the Instant Pot, then place a steamer inside. Put the bitter melons in the steamer, seamed side up.
Secure the lid, close the valve and cook for 2 minutes at high pressure.
Quick release pressure.
Remove the bitter melons and set them aside for now. Dump out the water from the pot and make sure to dry it completely.
Press the sauté button, adjust the heat to its highest setting, add the mustard oil and and wait for a few minutes to allow the mustard oil to get very hot (best if the screen says hot).
Add the bitter melons to the pot and fry them on all sides, flipping them over every few minutes once one side has browned. This step takes awhile, it should take a total of 15 or so minutes to get them fully browned.
Remove the kitchen twine and serve.