Heat half the oil in a large pan. Add the onions and celery and cook over a medium heat for 10 minutes until soft. Add the celeriac, apples and thyme and cook for 2–3 minutes. Add the stock, season, and simmer over a low heat for 30 minutes until the celeriac is tender.
Remove from the heat and blitz with a hand blender until smooth. Stir in half the crème fraîche . Heat the remaining oil in a pan and fry the sage until crispy. 3. Spoon the soup into bowls and top with the remaining crème fraîche . To serve, drizzle with extra-virgin olive oil and sprinkle with the crispy sage leaves and hazelnuts or grated horseradish.