First wash and pressure cook the chana dal until done. Remove the excess water from the dal and mash it nicely. Keep it aside.
While the dal is cooking, we can prepare the dough for the outer layer. To a large mixing bowl, add the all purpose flour, salt, baking soda, turmeric powder, oil and mix well.Now, add hot water little by little to the mixture and make it into a smooth and sticky dough. Take a little more oil and spread it fully around the surface of the dough. Cover with a wet kitchen towel and leave it to rest.
Add the jaggery to a small pan along with a little water. Cook until the jaggery is completely melted. Filter out the impurities using a metal filter.
Return the jaggery syrup to the pan and continue to cook in medium flame until it bubbles up.
Now add the grated coconut and mix well. Once the mixture slightly thickens, add the mashed dal to the pan and mix well.
Add the cardamom powder and ghee and mixture. Continue to cook until the mixture thickens and doesn't stick to the pan. Turn off the flame and cool it down to room temperature.
By now the dough would have become very soft and pliable. Take a small portion of the dough and roll it into a ball. Lay it out on a greased surface (I used a plate) and flatten it out using your fingers.
Take a small portion of the filling and roll it into a ball. Place the filling in the middle of the dough, use your fingers to pull the dough and cover the filling. Roll it back into a smooth ball. Place it back onto the greased surface and flatten it out using the fingers.
Heat a tawa (flat griddle) in medium flame and transfer the poli dough to the tawa. Drizzle oil / ghee and cook both sides of the poli until nicely golden.
Serve hot.