Heat half the oil in a large pan. Slice the onions and roughly chop the celery, and cook over a medium heat for 10 minutes until soft. Chop the celeriac, core and quarter the apples and add to the pan. Pick in the thyme leaves and cook for 2 to 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 leaves until crispy. 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.