White Sauce Splitting

Our answer

It is most likely that it is the cheese in the sauces that is causing the sauces to curdle, or split. In particular some types of Cheddar cheese can let out a fair amount of oil during cooking at higher temperatures and this won't mix very well with the sauce in the dish. Make sure that when you are making the sauce you add the cheese right at the end, once the pan of thickened sauce has been taken off the heat, and stir constantly until the cheese has melted as if the pan is still on the heat it is more likrly to overheat the cheese.

Also if you have recently changed the cheese you are using it is worth switching again to see if you get better results. And if you are baking the pie and lasagne at 200c/400F then reduce the oven to 180c/350F and bake the pie or lasagne for about 15 minutes longer (make sure they are piping hot in the centre before serving) as the gentler temperature may also reduce the risk of the sauce overheating in the oven.

For the fish pie if you are using raw fish and then pouring the sauce over before baking it could be that the fish is letting out some liquid as it cooks in the oven, which would also either thin the sauce or give it the appearnace of curdling. This may particulalry happen if the fish has been frozen and thawed. Poaching the fish first in milk will usually reduce this risk and the milk can then be used to make the sauce.