In a large saucepan, heat the oil over low heat. Tie the thyme sprigs and bay leaves together with a piece of string and add them to the pan with the onion and a pinch of salt.
Cook the onion for about 10 mins until softened.
Add in the garlic and cook for another minute, then add the celeriac and potato. Stir well and season to taste with salt and white pepper.
Next, pour in the stock, bring to the boil, lower the heat and then simmer for around 30 mins until the vegetables soft.
Remove the herbs, then stir in the cream. Remove from the heat and blitz with a hand blender until smooth. Stir through 1/2 tbsp. truffle oil at a time and taste for seasoning – the strength of the oil will vary, so it's better to start with less oil and add a little at a time.
Reheat the soup until hot if necessary. Serve in bowls topped with the hazelnuts, freshly ground black pepper and an extra drizzle of truffle oil.
