Heat 1 tablespoon of the oil in a large saucepan over medium heat.
Add the onions, apples and celery.
Cook for 10 minutes, stirring occasionally, until softened.
Add the sweet potato, parsnips, curry powder and pepper.
Cook, stirring occasionally, for 5 minutes.
Add the stock and tomatoes.
Cover and simmer for 20 to 30 minutes over low heat.
Taste for seasoning and add more curry powder, salt and/or pepper if necessary.
Being careful not to burn yourself, remove half the soup mixture and blend it with a stick blender, in a Vitamix or with a blender.
Return to the pan and heat through to serve.
Cool leftovers and store in the fridge. The soup improves with keeping, and will last 3 to 5 days in the fridge.