Cook the haddock and salmon in seasoned milk (onion, carrot, bay leaf, peppercorns and two cloves) until cooked.
Remove from the milk and, when cool enough to handle, flake the fish, discarding any skin or bone. Combine the haddock with the potato, butter, onions and anchovy essence in a mixer with a dough hook, then fold in the salmon, eggs, parsley and dill by hand until well combined (do not overmix).
Season to taste. If the mixture is too dry at this point add some of the haddock poaching milk.
Divide the mixture into four equal amounts. Dip in the flour, then the egg and finally the breadcrumbs and shape into patties. Refrigerate for two hours before use. Pan-fry in butter for five minutes each side, then keep warm in the oven.
Melt the 15g/1oz unsalted butter in a shallow pan. Once the butter is hot and foaming add the spinach and gently heat until the leaves have wilted. Remove from the heat and roughly chop the leaves.
To make the sorrel sauce, heat the fish stock until it has reduced by half, add the cooked sorrel and cream and gently heat until warmed through. Serve the fishcake on a bed of spinach and pour the sorrel sauce over the top.