Heat the olive oil in a pan over a medium heat. Add the chopped onion, celery sticks, garlic and grated fresh ginger and cook for 5 minutes.
Stir in the curry powder and cook for a further 5 minutes.
Add the parsnips and carrots and pour in the vegetable stock. Cover and simmer until the vegetables are tender – this will take about 30 minutes.
Purée the soup in batches, adding more stock to achieve the right consistency. Garnish with the cream and grated apple, if using, and serve.