Soup series: our buttermilk and coriander soup

This soup is one we first learned to make when we were visiting Zimbabwe, many years ago, in the 80's. It is a delicious, quick and unusual soup that was a speciality of a friend of ours called Nathoo, whose heritage was part African, part Indian, and it was based around one of the local products, a fermented milk product called Lacto, which is pretty close to buttermilk or kefir, as well as on coriander., and a curry spice mix--made fresh or dried if you like(though fresh tastes much the best). Buttermilk is easy to find in the supermarket.
Very quick to whip up, fresh-tasting and irresistible, this soup is good in both the cold and warmer months.
For the spice paste: coriander seeds, two cardamon seeds, a little fresh chilli. First fry the ingredients quickly in a pan(only a few instants) then grind either in a mortar and pestle or a machine. Set aside. Chop a largish onion finely, fry, add crushed garlic, the spice mix, salt, chopped coriander leaves(reserve some for end for decoration), add a little stock of your choice--and then buttermilk--about half a cup of buttermilk to a cup of stock. Cook for only a couple of minutes, then serve, with reserved chopped coriander on top. Et voilà!