1) Remove the potatoes from the oven and mash.
2) Place in a bowl. Pour the milk and butter into a pan and bring to a simmer. Mix in the potato slowly, using a spatula. Season.
3) Make the bechamel sauce. Melt the butter in a pan and slowly add the flour. Cook for 5 minutes, stirring with a spoon. Gradually pour in the milk a little at a time, stirring continuously until you obtain a smooth velvety sauce. Season to taste.
4) Place the pieces of fish and peas into an oven proof dish around 20-by-20cm. Season and add a squeeze of lemon juice. Pour over the bechamel sauce. Spoon on the mash, starting at the edges and working towards the centre. Bake in the oven for 40 minutes.