Preheat the oven to 220C/200 Fan/Gas 7
Gut, scale and remove the fins from the whole turbot. Rinse and pat dry on a piece of kitchen paper.
Melt 60g/2¼oz of the butter in a small saucepan and gently cook the vegetables for about 4 minutes, adding the white wine halfway through. Season with the salt and pepper and leave to cool a little. Stuff the cavity of the fish with most of the vegetables, leaving some to rest the turbot on when you roast it.
Butter the turbot with the remaining 30g/1oz of butter. Season well with salt and pepper and place the whole fish in a roasting tin with some of the vegetables underneath. Cook in the centre of the oven for about 30 minutes. Test whether the fish is cooked by cutting down on to the backbone at the thickest part and checking that the flesh has turned white right through.
For the sauce, put the chopped onions, vinegar, white wine and fish stock in a small pan, and boil until all but a couple of spoonfuls have evaporated. Add the butter, then the sorrel and spinach and cook for a minute soften. Put the egg yolks into a blender. With the blender running, pour the contents of the pan through the small hole in the lid. Season and pour the mixture through a sieve into a warm sauce boat.
When the turbot is cooked, transfer to a serving dish, garnish with sorrel, and fillet it at the table, giving each person some of the roasted vegetables from under the fish as well as the vegetable stuffing. Serve with the sorrel sauce.