Peel and chop the parsnips and potatoes into 3-4 cm cubes and put them in a pot with about a half-litre of water. Bring to the boil then simmer until you can break the pieces easily with a fork. Drain and save the water for stock. Puree the vegetables by pushing them through a sieve with the back of a ladle or a big spoon. If you don't have a sieve, mash with a fork or a potato masher.
Chop the onion finely and fry it in a little oil until it begins to go golden and add the garlic, crushed or chopped, and the powdered spices. Stir it all around for a minute, then add the vegetable puree and stock. Thin with water until you get a consistency you like then bring it to the boil and add the stock cube. Simmer gently for about 10 minutes adding some salt along the way if you think it's necessary. When serving, garnish with a little green coriander or a sprinkle of garam massala.