First make the sauce. In a small saucepan set over medium heat, melt the butter. Whisk in the flour until a paste forms and cook for a further minute or so. Stir in the mustard and milk. Continue whisking until a smooth sauce appears. this will only take a minutes. Add parsley, salt and pepper. Set aside
In a medium saucepan of boiling water, add the potatoes and cook until soft. This will take about 15 to 20 minutes. When cooked, remove from heat, drain and set aside
Meanwhile, in another medium saucepan of boiling water, add the cod fish and boil for about 10 minutes. Remove from heat and drain the water. Leave to cool so you can handle it.
Once the fish is cool enough to handle, remove the skin and you will notice that the fish will just flake apart. Remove any bones that you find in the fish and discard them.
In a large mixing bowl, add the potatoes and roughly mash with a fork. Now add the fish, onion, parsley, oregano and salt & pepper. With a wooden spoon, stir to combine. When it is all combined you can start making your fish cakes.
Evenly divide your mixture into 12 portions. Using both hands, form a ball and then gently flatten each one slightly until they take the desired shape. Repeat these steps for remaining fish cakes.
Next, on a clean working surface, spread out the bread crumbs. With one fish cake at a time, completely coat it in the bread crumbs and put on a clean plate. Repeat for further fish cakes.
In a large skillet set over moderate heat, add the oil and bring up to temperature. When you put the fish cakes in the pan you want them to instantly sizzle. Cook for about 5 minutes or until it is golden brown and flip over. I usually use a silicone spatula to do this, and I have more control in flipping them. Continue cooking the other side for the same amount of time, or until golden brown. Repeat for whatever fish cakes you have left.
Serve immediately topped with the parsley sauce.