Make sure the fish has no bones and then place into a medium but deep frypan. Pour in the milk and bring to a simmer over low heat and cook until the fish is cooked through, approx 10mins.

Carefully remove fish and place on a plate and pour the milk into a bowl and reserve.

In a saucepan, add the butter and melt over gentle heat. When completely melted, add the flour and stir over low heat until all combined. Increase the temperature slightly and cook the flour/butter mixture for an additional minute stirring constantly. 

Add the reserved milk a little at a time and stir constantly with a whisk over medium heat. As it heats the mixture will thicken. Once all the milk has been added add the cheese and stir through until it’s completely melted. The mixture should be thick but still able to be poured.

At this stage add the raw peas, parsley and dijon mustard and season with salt and pepper to taste.

Carefully break the fish apart with your fingers into pieces, allowing the way the fish naturally breaks up to guide you. Do not shred the pieces. They should end up being approx 2cm long by 6mm wide sections of fish.

Carefully stir the fish pieces into the cheese mixture making sure you dont over stir so as to not break the fish up any further.

Place the mixture into a bowl and cover with cling film and place into the fridge until completely cold.

Preheat oven to 180C

Once the fish mixture is cold, pour into a 21 x 15cm oven proof pie dish with a flat edge.

Whisk the egg yolk well and using a pastry brush, paint the egg yolk onto the exposed flat top rim of the pie dish.

Cut a piece of puff pastry just big enough to cover the entire pie dish and place on top of the dish covering the fish mixture and the edges of the pie dish.

Using a small sharp knife, trim the pastry so it is neat against the pie dish rim. Using a fork, make fork marks along the rim, pushing the pastry against to pie dish to make it stick.Using a small sharp knife, prick the centre of the pastry in two places, this will allow steam to escape.

Using the left over pastry and a small round pastry cutter, cut half circles of pastry and place them on top of the pie (excluding the rim) overlapping the round edge over the previous circles flat edge slightly, making them resemble fish scales.

Once you have finished decorating the pie, add a few drops of milk to the remaining egg wash and mix well and brush the egg wash on to the top of the pastry using a pastry brush making sure all the pastry is covered. 

Place in the oven and cook for 30 – 40mins until the pastry is golden brown and the pie filling is hot. It is best to place it on a tray into the oven so if the pie filling bubbles over the tray is there to catch the overflow.

If the pastry starts to get to brown, cover the pastry loosely with alfoil.

Serve hot.