Sprinkle salt on one side of the hake and set aside.
Melt butter in a saucepan and fry onion and garlic until softened.
Stir in 200g of button mushrooms and cook for two minutes.
Add sherry and dried mushrooms, then simmer until liquid evaporates.
Pour in double cream, boil, and simmer until reduced by one-third.
Blend the mixture with yoghurt until smooth and strain into a clean pan.
Simmer celery root in 200ml water with butter and salt for 25 minutes.
Baste celery with pan juices and sprinkle with thyme leaves.
Salt the remaining mushrooms and set aside for five minutes.
Coat the hake in flour and fry in oil until golden on one side.
Flip fish, add butter, and fry until the butter browns.
Add vinegar and thyme to the pan and baste the fish.
Remove fish and stir sugar and celery leaves into the pan sauce.
Plate the purée, celery, and hake, then garnish with mushrooms and hazelnuts.