Heat a small frying pan and dry roast the coriander, cumin and cardamom seeds. This is to toast them and draw out their flavour.
After 2-3 minutes, they will change colour and start to jump in the pan. Remove them from the pan and crush them finely with a pestle and mortar.
Heat the butter and oil in a pan until the butter begins to foam, then add the onions, for another two minutes, then add all the crushed spices, along with the turmeric and ginger, stir and let it all continue to cook gently for a few more minutes while you peel and chop the parsnips into 2.5cm/1 inch cubes.
Add the parsnips to the pan, stirring well, then pour in the stock, add some seasoning and let the soup simmer as gently as possible for one hour without putting on a lid.
When the soup has been simmering for an hour, remove it from the heat, then liquidise it if possible. If not, use a food processor and then sieve, squashing the ingredients through using the bowl or a ladle.
After the soup has been pureéd return it to the pan and taste to check the seasoning. When you're ready to serve, re-heat very gently.