Preheat the oven to 200°C/400°F/Gas Mark 6. Check the fish carefully and remove any bones, then rinse in cold water and pat dry. Bake the cod and salmon in foil with a bit of olive oil for 12-14 minutes or until the fish is cooked through. Take the fish from the foil, flake the flesh and set aside.
Boil the potatoes for about 20 minutes, until tender. Drain, allow to steam briefly in the warm pan then mash with half the butter. and put to one side.
Melt the remaining butter in a saucepan and add the onion and garlic. Cook until soft, for 4-5 minutes, then transfer to a mixing bowl and add the mashed potato, flaked fish, egg yolk, parsley and lemon juice. Mix together well.
Divide the mixture into 8-10 pieces and shape each one into a fishcake. Put the flour seasoned with 1/4 teaspoon of freshly ground black pepper into one shallow dish, the beaten egg into another and the breadcrumb mixture into a third. Coat each fishcake first in the flour, then the egg, then the breadcrumbs.
Melt the oil with the butter in a pan over medium heat. Cook the fishcakes until golden brown on both sides and warmed right through to the middle: about 8-10 minutes.