Preheat the oven to 180 degrees. Place the potatoes in a saucepan of water and bring to a boil , reduce to a simmer and cook until tender.
Meanwhile add the butter and flour in a saucepan and allow the butter to melt on a low heat. Stir the butter and flour together with a wooden spoon to create a roux (this will act as a thickening agent for the sauce).
Start to gradually add the milk. Keep stirring until a thickened sauce is made. Season with salt and white pepper.
Mash the potatoes with the butter , milk , salt and white pepper.
Add the fish, prawns peas, lemon zest and parsley to the white sauce and stir to combine. Pour this into a medium pie or oven proof dish and top with the mashed potatoes. Drizzle some olive oil on top of the mashed potatoes.
Bake in the oven for 35-40 minutes.