Pre-heat the oven to 355 °F/180 °C

Pour the fish stock and milk into a large saucepan and bring to a gentle simmer. Add the fish pieces and and poach for 5 mins.

Remove the fish pieces with a slotted spoon and keep to one side. Reserve the liqueur.

Melt the butter in a medium sized saucepan over a medium heat. Add the sliced leeks and cook for 5 minutes until the leeks are soft.

Whilst still hot, add the flour and stir well with a wooden spoon. Pour the fish liqueur into the pan and stir again, raise the temperature and cook for 3 minutes until the sauce is slightly thickened. Turn the heat off. Add the fish and season with salt and pepper. Leave to one side.

Place the fish and sauce into an ovenproof dish, cover with a thick layer of mashed potato fluffed up with a fork. Sprinkle with Grated Cheese if using.

Put the dish onto a baking sheet and cook in the center of the preheated oven for 20 - 30 minutes or until the sauce is bubbling beneath the potatoes. Serve immediately.