Place the grated celeriac into a saucepan with the milk, salt and bay leaf and bring to the boil. Turn down the heat and simmer for 30 minutes, stirring regularly. Pass the mixture through a colander, discard the celeriac and then pass the milk through a fine sieve into a clean saucepan. Transfer the infused milk into a shallow dish.
Place the dish on a wire rack. Set the rack over oak wood chips in a home smoker or kettle barbecue, cover tightly with a lid and set over a medium heat for five minutes.
Remove the milk from the smoker and measure 500ml/17fl oz of liquid. Transfer the milk to a medium-sized saucepan. Add the agar agar, bring to the boil, whisking continuously, for two minutes. Pour the contents of the pan into a shallow dish and set aside to cool and set.
Once cooled and fully set, transfer the mixture into a food processor and blend until a smooth purée. Season, to taste, with salt, transfer the mixture to a bowl and cover with clingfilm. Chill in the fridge until required.
Preheat the oven to 150C/300F/Gas 2.
For the hazelnut and parmesan pesto, place the hazelnuts on a roasting tray and roast in an oven for 10 minutes.
Remove from the oven, transfer to a stainless steel bowl and crush to a fine crumb with a rolling pin. Add the parmesan, the oils, salt and freshly ground white pepper. Stir until well combined and set aside.
For the baked celeriac, peel the edges to neaten the pieces of celeriac and place into a bowl. Mix the two oils and add to the bowl of celeriac. Stir until all the celeriac pieces are evenly coated and add a generous pinch of salt and freshly ground white pepper.
Place a large sheet of aluminium foil onto a work surface and cover with another equally sized piece. Place the celeriac into the centre of the foil. Fold one end over to the other and fold over the meeting ends to seal. Fold over the side edges to create a sealed pouch.
Transfer the pouch onto a baking tray and bake in for 40 minutes. Remove from the oven, open the pouch and check that the celeriac is tender by inserting a skewer or toothpick.
Place a large heavy based saucepan over a medium heat for one minute. Add the butter, allow it to melt and then add the cabbage and a pinch of salt. Cook for 3-4 minutes, stirring frequently, until the cabbage softens.
Add 100ml/3½fl oz chicken stock, cover and cook for a further 3-4 minutes, or until the cabbage is just tender. Set aside at room temperature.
Place two large heavy-based frying pans over a high heat for one minute. Add two tablespoons of grapeseed oil to each. Season the skin-side of the turbot with salt and fry the fish for 1-2 minutes until golden-brown.
Add half the butter to each pan, turn the fish over, add 75ml/3fl oz of chicken stock to each pan and transfer to the oven. Cook for three minutes alongside the celeriac.
Remove the fish from the oven, coat each portion with a spoonful of pesto and set aside to rest for five minutes.
Heat through the celeriac milk purée and the cabbage.
Lay out eight large pre-heated plates. Place a generous spoon of purée at the top of each plate and drag the spoon through it to smear the purée around half of the plate.
Unwrap the baked celeriac and transfer a piece to each plate.
Place a flat bed of cabbage in the centre of the plate, sit a piece of turbot on top and spoon some of the buttery juices around the fish.