Melt the butter in a large, thick-bottomed pan and gently cook the onion, leek and parsnips until soft.
Add the cumin, garlic, ginger and thyme and cook for a further 2-3 minutes.
Pour in the stock, bring to the boil, season with salt and pepper and simmer for 30 minutes.
Process the soup in a blender, and strain it through a sieve.
Return it to a clean pan, pour