Melt the butter with the oil in a large saucepan, add the onion, garlic and ginger and cook over a medium heat for 4–5 minutes until softened.
Stir in the curry powder and cumin seeds and cook, stirring, for 2 minutes, then stir in the parsnips, making sure that they are well coated in the spice mixture.
Pour over the stock and bring to the boil, then cover and simmer for 20–25 minutes until the parsnips are tender. Season to taste with salt and pepper.
Blend the soup with a stick blender until smooth, or transfer to a food processor or blender, in batches, to blend. Reheat gently if necessary.
Serve in cups with dollops of natural yogurt, garnished with the coriander and with warmed naan bread.