Put the potato, cauliflower, and onion into a large saucepan with the oil anf 45 ml of water. Heat until hot and bubbling, then stir well, Cover the pan and turn the heat down. Continue cooking the mixture for about 10 minutes until potato is tender.
Add the garlic,ginger, and spices. Stir well, and cook for another 2 minutes, stirring occasionally.
Pour in the stock and season to taste. Bring to a boil, then cover and simmer for about 20 minutes.
Puree with your stick blender. Stir in the yogurt, adjust the seasoning, and serve with coriander.