Heat the oven to 180C/160C fan/gas 4. Layer a large sheet of baking parchment over a large sheet of foil, then put the celeriac in the centre. Rub all over with the truffle oil, then season well. Scatter over the garlic and sage leaves, then bring the edges of the parchment and foil up, scrunching together to seal make a parcel. Put in a roasting tin and roast for 2 hrs, or until tender and golden. Baste halfway through with an extra drizzle of truffle oil, if you like.
Meanwhile, make the sauce. Melt the butter in a saucepan until foaming, then add the flour and stir until smooth. Cook for 2 mins, then whisk in the milk, a little at a time, until fully incorporated and smooth and creamy. Whisk in the cheese and cream, then season to taste.
Cut the roasted celeriac into four wedges. Spoon over the cheese sauce and scatter with the hazelnuts.