Preheat the oven to 165°C.
Salt the cod, wrap in cling film for twenty minutes, then wash and halve.
Boil potatoes until soft, drain, and dry in the pan.
Discard any open cockles that do not close when tapped.
Steam cockles with parsley stalks, shallots, and wine in a hot pan until open.
Drain cockles and pass the reserved stock through a muslin cloth.
Melt butter and stir in flour to make a roux, then whisk in stock, cream, milk, and crème fraîche.
Season the sauce with salt, cayenne, and lemon juice.
Sear cod in butter in an ovenproof pan, then roast in the oven for eight minutes.
Add shelled cockles and chopped parsley to the sauce and keep warm.
Rice the potatoes and fold in melted butter and cream.
Serve the cod and sauce over the mashed potatoes.