Slice the button mushrooms very thinly, then fry in 100g of the butter, dice 20g of the butter very small and put into the fridge to set hard. Drain the mushrooms of any excess butter and place into a small sauce pan, cover with cold water (not too much or your sauce will be diluted), bring to the boil then take off the heat and cover the pan with cling film. Leave for at least 4 hours, then strain the liquid, bring back to the boil, add cream and the 20g of hard butter. Boil once but only for 10 seconds, season with salt, pepper and keep warm.
Remove the crust from the bread and cut into 4. Pick the parsley to remove stalks, then wash and dry. In a food processor blend the bread and the parsley together for about a minute. Then add cheese followed by the softened butter, blend until a smooth green paste is formed, finally fold in the coco nib and hazelnuts, roll the paste out between 2 pieces of greaseproof paper to about 5mm thick. Then place into the fridge to set hard.
Once hard, put each piece of brill on top of the crust still with the paper on, then cut around each piece of fish leaving some extra at each side. Then remove the paper from the crust and place on top of the fish. Transfer the crusted fish onto a oven tray with greaseproof paper under, pre heat your oven to 180ºC. Then cook for about 6-7 minutes (check the fish is cooked by gently inserting a cocktail stick into the thickest part of the fish. If you do not feel any tension, it is ready).
Sauté the wild mushrooms and spinach in the remaining 20g of butter, (separately), drain onto a paper towel, place the spinach in the centre of a warm bowl with the wild mushrooms in a ring around it. Then using a whisk or electric hand blender, whisk up the warm mushroom sauce, spoon the bubbles around the plate over the spinach and mushrooms. Then place the fish on top of the spinach and serve immediately.