First, Place the potatoes in a large saucepan and barely cover with water, cook for 25 minutes until very tender but not falling apart on the simmering stovetop. Alternatively, bring to the boil for 3 minutes, drain all the water and place with a lid in the simmering oven for 30 minutes or so until steamed.
Meanwhile, on the boiling stovetop, heat a large saucepan with the butter and add in the leeks and onion and sauté gently for a 1 – 2 minutes until they are softened completely.
Add in the cornflour and cook for a further 1 minute. Season well.
Pour in the white wine and pour in the milk and wholegrain mustard. Stir well and allow to come to a gentle boil.
Transfer to saucepan on the simmering stovetop. Cook for 2 – 3 minutes, then add the fish. Gently stir once or twice and bring to simmer for a further 5 minutes.
Remove from the heat and stir in the parsley. Check the seasoning with salt and pepper.
Drain the potatoes and return to the saucepan over a simmering stovetop. Using a potato masher roughly mash the potatoes. Add the butter to the potatoes, and season with salt and pepper. Beat with a wooden spoon until fluffy.
Spoon the mash on top of the fish mixture and place in the baking oven for 20 -25 min or until the fish is cooked through and the top of the pie is bubbling and golden brown.
Serve with some green salad leaves.