First make the tartar sauce. Whisk together all the ingredients and set aside.
In a medium saucepan of boiling water, add the potatoes and cook until soft. This will take about 15 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.
In a medium skillet, add one tablespoon of olive oil and the leeks. Cook the leeks over a medium heat for about 10 minutes, stirring occasionally until they are soft and lightly golden. Remove from heat.
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 the two tablespoons of milk and mash really well. Now add the fish, leeks, parsley, oregano and salt & pepper. With a wooden spoon, stir to combine. When it is all combined you can start making your fish cakes.
Form small balls out of the mixture, 10 give or take. On a lightly floured surface, flatten out the fish cakes into disks. Repeat for all fish cakes then coat completely in flour.
In a large skillet set over moderate heat, add two tablespoons of the olive oil and bring to a high heat. When you put the fish cakes in the pan you want them to instantly sizzle. Cook for about 6 minutes or until it is golden brown and flip over. Continue cooking the other side for the same amount of time, or until golden brown. Repeat for whatever fish cakes you have left. Use the remaining olive oil to cook the rest of the fish cakes.
Best served immediately. Can be reheated in the oven for best results.