Cook the arbi corms in boiling water until a knife inserted through the middle goes through cleanly and without any resistance. I usually cook the arbi with some water in a pressure cooker, which is the easiest way to get them done, but you can submerge them in water, bring to a boil, cover with a lid, and let them simmer about 10-15 minutes until they are done.
Peel the arbi, chop roughly, and place in a food processor with half a cup of water along with the powdered spices, salt and the mint. Process until you have a really smooth paste. If you don't have a food processor, try mashing the arbi with a potato masher as smooth as possible, and then mix in the water and the spices.
Mix the water chestnut and chickpea flours with the arbi paste and add the jowar a little at a time until you have a firm dough that does not stick to your fingers. You can do this by hand or in a stand mixer with the dough hook attached. Once your dough is ready, immediately form 12 balls and roll them in your palms to make them really smooth.
Now liberally flour the rolling surface and your fingers and pat out the parathas using the your fingers. You want to spin the paratha slightly each time your fingers make contact with the dough. It may sound complicated but trust me, you will get it. If the paratha feels like it's sticking, flour again. If you do end up with a tear, patch it, flour over it, and continue patting it out. This paratha should be no more than five inches in diameter.
Heat a flat griddle over medium-high heat. Place the paratha on the griddle and flip over when bubbles start to form and golden-brown spots appear. Spray on some oil or and when the underside has golden-brown spots, flip over once more and cook for a few more seconds.
Serve hot with some vegan raita, chutney, or spicy Indian pickles.