First make sure the fish is skinned and boned and chopped into chunks.
Place the fish into a saucepan and add the wine and water. Bring to the boil and simmer for 5-6 minutes until the fish is just cooked. (Do not overcook)
Remove the fish to the dish (4 individual ovenproof ramekins or large casserole dish ) and reserve the poaching liquid.
In another saucepan, melt the butter and add the flour. Whisk for a few minutes and allow the flour to cook away any raw flavour.
Add the poaching liquid slowly, whisking all the time and continue cooking on a medium heat until the sauce resembles the consistency of custard. If the sauce is too thick whisk in some cream or milk until it thins out. Taste the sauce for seasoning. If you are using smoked fish it may be salty enough.
Then add in the grated cheese whisking until melted in. This will thicken the sauce slightly. Turn off the heat and set the sauce aside.
Saute the carrots and leeks in the olive oil or butter if you prefer. After a couple of minutes add the leeks and soften the veg for about 5 minutes depending on the size of the dice. When cooked, with still a little bite, add to the cheese sauce.
Pour the sauce over the fish, covering well.
Roll the pastry out until it is about 1/2 cm thick. Cut out a round slightly bigger than the dish you are covering.
Brush the rim of the bowl with beaten egg and cover with the pastry disk. Brush the pastry with the egg wash and put a small slit in the top to allow the steam to escape while cooking.
Bake in the oven for about 20 mins for individual pies or 30 - 40 mins for the large pie. Or until the pastry turns golden brown.
Cool for a few minutes before eating, especially if it is for children as it will be very hot.