Steam the potatoes until tender – about 15-20 minutes, depending on their size.
Meanwhile, put the milk and bay leaf in a deep frying pan and bring to a simmer. Season with salt and add the cod, making sure there is enough milk to cover.
Poach gently for about five minutes, until the cod is just cooked through, then transfer to a plate to cool, reserving a cup of the poaching milk.
Mash the potatoes with a masher or potato ricer. Pour in four tablespoons of the poaching milk and a third of the butter, and continue to mash until smooth, adding a splash more milk if needed.
Flake the cod and salmon, stir in with the spring onions, parsley and egg yolks and thoroughly mix and season. Shape into 12 even patties and refrigerate to set while you make the sauce.
To make the sauce, melt the butter in a small frying pan over a medium heat and gently saute the shallots with a pinch of salt for five to seven minutes, until soft, adding the flour for the last minute of cooking.
In a separate bowl, whisk together the mustard and the yolks, then slowly whisk in the creme fraiche.
Add to the softened shallots and simmer gently (don’t let it boil or the eggs will curdle) for five minutes, stirring, until the sauce thickens. Take off the heat, stir in the wild garlic, lemon and capers, and season. Keep warm.
When you’re ready to eat, generously coat the fish cakes in the flour. Melt a third of the butter and a small splash of oil in a large frying pan and cook four of the fishcakes at a time, turning over once golden – about two-three minutes on each side.
Check they’re cooked through and either serve at once while you cook the remaining cakes, or keep warm in a low oven. Serve with the warm sauce, some wilted spinach or greens and a lemon wedge.