Why does my white sauce curdle?

[Tigana]

Milk, flour, butter, potentially cheese. Fine in the pan. But recently, once added to pie/lasagne etc and popped in oven for 30 mins, it has been curdling [mild irritation].

did you cook the flour out completely?

?????
Am very amateur cook.

Do you mean, was it lumpy?

how do you make it?

do you start with a roux?

no, flour needs to be completely cooked, when i make white sauce i melt the butter and then i cook the flour in the butter (ie make a roux) I do this for 5-10 mins until little craters appear in the roux (looks a little like a honeycomb) then I add the milk/milk and cheese and cook for a bit longer. This ensures the flour is cooked through.

what bewilder said.

The recipe says to put milk in pan, add butter and flour and stir lots.

Also a bit of cornflour helps it stay together.

I'd suggest using bewilderbeast's method

Did you use skimmed milk? It splits more easily than semi or full fat.

I will convert to the patented bewilderbeast method for white sauce!
Thank you!