First, make the mornay sauce at least a few hours before making the fishcakes. Put the flour and butter in a small pan over a low-medium heat and cook until the butter is melted, then stir to make a paste. Slowly whisk in the milk until you have a smooth sauce. Continue cooking until thickened, about 5 mins, then remove from the heat and stir in the cheese and mustard. Season. Oil a few small pieces of baking parchment, then use to line four holes of a muffin tin. Alternatively, use silicone cupcake cases. Divide the sauce between the cases and freeze for at least 2 hrs.
Put the potatoes and bay leaf in a large pan, cover with water and season with salt. Bring to the boil, then reduce the heat and simmer for 15-20 mins until tender. Remove the potatoes with a slotted spoon, put in a colander and leave to steam-dry.
Add all the fish to the water in the pan, reduce the heat to low and poach for 5 mins. The fish is cooked when it just starts to flake apart. Carefully transfer the fish to a board with a slotted spoon, and leave to cool for 5 mins.
Flake the fish, discarding any skin and bones. Tip the potatoes into a clean pan, season well and grate over some nutmeg. Mash until smooth, then fold through the fish, parsley and lemon zest. Leave to cool.
Divide the fishcake mixture into four portions. Working with one portion at a time, mould into a ball. Make a well in the centre, and push one of the frozen mornay sauce discs into it. Shape the fishcake around the sauce middle so you have a smooth hockey-puck shape. Repeat with the remaining portions and sauce, then freeze the fishcakes for at least 15 mins.
Put the flour in a shallow bowl, the egg in a second, and the fresh breadcrumbs in a third. Season the egg. Coat the fishcakes well in the flour, then the egg, and finally the breadcrumbs. If you have any egg or breadcrumbs left at the end, keep dipping and coating until everything is used up. Freeze the fishcakes until firm, about 1 hr. Will keep frozen for up to one month. Defrost in the fridge for 2 hrs before cooking.
Fill a large, deep pan no more than two-thirds full with oil, and heat it to 180C. Or, use a deep-fat fryer. Heat the oven to 190C/170C fan/gas 5. Deep-fry the fishcakes for 7 mins, turning halfway, until crisp. Drain on a plate lined with kitchen paper, then transfer to a baking sheet and bake for 5 mins (or 15 mins from frozen) until the middles are hot. Serve with salad and lemon wedges.