Heat oil in a heavy stockpot or Dutch oven over medium heat. Add leeks and cook until soft and translucent, about 8 minutes. Stir in garlic and bay leaf, cooking for 1 additional minute. Add celeriac, potatoes and apples; cook until soft, about 10 minutes.

Season with salt and pepper and deglaze the pot with wine, cooking until almost evaporated. Pour in enough vegetable stock to cover and bring to a boil. Lower the heat to a simmer and cook, partially covered, until celeriac and potatoes are very tender, about 30 minutes.

Remove bay leaf and let soup cool slightly. Carefully puree soup until smooth (either with an immersion blender or in batches in a blender or food processor). Return soup to the stove to reheat, adding additional stock if too thick. Taste and season with salt and pepper, if needed. To serve, drizzle with truffle or olive oil.