Wash and peel the potatoes and cut into 3cm cubes. Boil in salted water for 12-15 minutes until tender. Once cooked, drain the water and mash the potato until smooth. Add the butter and milk and stir in.
Cut up the fish into chunks of 2cm cubes and set aside.
To make the filling: Finely slice the leeks and place into a pan with the butter, sprinkle in the flour and stir continuously on a medium heat for 2 minutes. Add in the milk whilst still stirring gently and bring to the boil and simmer for 4 minutes or until thickened. Remove from the heat, add in the fish, peas, mustard, a pinch of salt and a generous helping of freshly ground pepper. Add in half the cheese, stir then empty into a deep baking dish.
Spoon the mashed potatoes onto the mixture and gently even out across the dish. Sprinkle the remaining cheese on top then place in a preheated oven at 200c/180c fan/gas mark 6 for 25-30 minutes or until golden and edges are bubbling.
Scoop out with a large serving spoon onto a plate and eat.