Boil potatoes for 15 minutes or until tender, then drain and mash.
Stir parsley, lemon zest, and juice into the mash, then fold in salmon and haddock.
Enclose a quail's egg inside the mixture to form four equal balls.
Heat oil in a large saucepan or deep-fat fryer to 180°C.
Prepare three bowls with flour, beaten egg, and breadcrumbs.
Dredge balls in flour, dip in egg, coat in breadcrumbs, and fry for 2 minutes.
Drain fishcakes on kitchen paper using a slotted spoon.
Boil shallots, vinegar, and wine until reduced to 30ml.
Reduce heat and whisk in 225g butter gradually until incorporated.
Strain the sauce through a sieve and stir in chives.
Cook spinach in a hot pan with 50g butter and 30ml water for 3 minutes.
Serve fishcakes on the spinach and top with the butter sauce.