Begin by baking all the cod fillets, with a little salt, until they're cooked through and flake into pieces. Press into a square brownie tin and freeze until solid.
While it's freezing, make the bread. Preheat the oven to 180°C. Bloom the yeast and sugar with the warm milk for around ten minutes, add eggs, then, using a mixer (or your hands!) knead it into a smooth dough. Knead in the softened butter and place it in a 23cm cake tin, allow it to rise for 30 mins then bake until cooked through and a light brown on top. Allow to cool before slicing in half.
Lay half the cheese in a square and lay the other half on top then roll together between sheets of clingfilm to make a massive cheese slice.
Mix together the ingredients for tartar sauce.
Take the frozen fish square and dip it a large bowl containing the flour, then dip in the eggs followed by the panko. Repeat for an extra crunchy exterior.
Fry in hot oil until golden and crispy all over, then remove, drain excess oil and build up the burger! First lay down the cheese slice, then the crunchy fish square, then finally the top bun. Get a good slice, yum!