1. To make the pakoras, gradually add buttermilk (or yoghurt, plus water as needed) to the besan to make a thick smooth batter. Add turmeric, red chilli powder, chopped onion and potatoes, and mix. Heat the oil in a karahi till just below smoking. You can test by putting a drop of the batter into the hot oil -- it should sizzle and rise to the top but not get browned right away. Add the baking soda, and mix well. Drop batter by spoonfuls (I use a teaspoon) in batches to make small pakoras, not more than ¾ inches across. Fry till medium brown, and drain on a paper towel.
2. Do not add salt to the pakora batter for two reasons. One, it supposedly keeps them from sucking up too much oil. Two, and more important, it ensures that you will have pakoras for the kadhi. Like cake, you cannot eat your pakoras and have them too :D !
3. Baking soda makes the pakoras light and soft. If you want a lower sodium version, and wish to avoid baking soda, beat the batter till light, and then add the chopped onions and potatoes. Fry similarly in hot oil, and soak in a bowl of water immediately. Tip the pakoras with this water into the kadhi.
4. Mix the other cup of besan with the remaining buttermilk (or sour yoghurt). Add water to thin. If you see any lumps, just let the mixture stand for a few minutes and then stir again; the lumps will dissolve.
5. Retain just 1 tablespoon of oil in the karahi. To the hot oil add the following, in order: cumin, mustard, nigella, and methi seeds, hing, and the whole red chillies. Stir and add the turmeric and red chilli powder. Give the besan-buttermilk mix a good stir and pour into the karahi. Turn the heat to medium, add salt, and stir. The kadhi will begin to thicken. Add more water if needed; the consistency should be that of very thick creamy soup.
6. Bring the kadhi to a boil, add the pakoras, and stir. Turn the heat down to a simmer. Cover and cook for 20 minutes to half hour, stirring occasionally to keep it from sticking to the bottom. Traditionally, the kardhi would bubble away on the very low heat of an angeethi for hours, thickening gradually. But it is not an implement that could survive the fast pace of city life. In the villages they might still use it on occasion.
7. Transfer the kadhi to the serving bowl. For the final flourish, just before serving, heat a teaspoon of ghee. To it add cumin and red chilli powder, and pour it over the kadhi.
8. Serve hot with rice. It is good on its own too. I usually polish off a katori or two before it makes it to the table