In a large saucepan, heat the oil over a 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 for about 10 mins until softened but not coloured.
Stir in the garlic and cook for 1 min more, then tip in the celeriac and potato. Give everything a good stir and season with a big pinch of salt and white pepper. Pour in the stock, bring to the boil, then simmer for around 30 mins until the vegetables are completely soft.
Discard the herbs, then stir through the cream, remove from the heat and blitz until completely 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.
To serve, reheat the soup until piping hot, then ladle into bowls and top with the hazelnuts, some black pepper and an extra drizzle of truffle oil.