First, Place the potatoes in a large saucepan and barely cover with water, cook for 25 minutes until very tender but not falling apart.
Meanwhile heat a large saucepan with the butter and add in the leeks, carrot and onion and sauté gently for 2-3 minutes until they are softened completely. Add in the corn flour if using and cook for a further 1 minute. Season well.
Pour in the white wine and bring to simmer stirring to avoid lumps from the cornflour.
Next, pour in the milk or cream and wholegrain mustard and allow to come to a gentle boil. 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 low heat. 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 oven for 20 -25 minutes or until the fish is cooked through.
Serve with some green salad leaves.