1. Heat oven to 190c / 175c fan
2. Cover the celeriac with water and parboil for 5 min. Drain and allow to steam.
3. Place the celeriac on a baking tray, drizzle with olive oil, season well and roast for 10 - 15 min. or until tender and nicely caramelised. Remove and allow to cool.
4. Reduce the temperature to 180c / 160c fan.
5. In a very large bowl, whisk together the cream, eggs, sweet onion relish, mustard, half the cheese, half the thyme, truffle oil and season well.
6. Add the cooled celeriac and brioche to the egg mixture and leave for 10 min.
7. Heat the butter in a shallow pan and cook until the butter turns brown. Add the hazelnuts and remaining thyme and cook for 2 min.
8. Evenly pour the celeriac and brioche mix into an ovenproof dish. Spoon the hazelnut & thyme butter over the top and sprinkle over the remaining cheese.
9. Cover the dish with foil, and cook in the oven for around 20-25 min., or until it puffs up and starts to caramelise.
10. Add a drizzle of truffle oil and a sprinkling of fresh thyme before serving.