Pour the milk into a saucepan and add your stock cube or powder, bring to the boil
Poach your fish in the milk until cooked. How long depends on the thickness of your pieces.
Remove the fish from the milk and set aside.
In a clean saucepan, melt butter and add flour. Let simmer for 2 minutes to cook away the flour taste.
Gradually add the milk whilst stirring. Once it has thickened, check consistency. You may want to add more milk if you think it's too thick. Once you're satisfied with the sauce, let simmer for a couple of minutes.
Add the parsley and season with salt and pepper. Then add the chopped egg.