I recently got a question from Phyllis who asked, “Every time I make scalloped potatoes my sauce breaks. What am I doing wrong.” This seems to be a frequent problem with scalloped potatoes particularly when you use a recipe that calls for layering the potatoes, onions, and cheese and pouring unthickened milk over them. Heat can cause milk to curdle and the lower the fat in the milk the more likely it is to happen, but there are things you can do to avoid it.
The best way to prevent curdling in scalloped potatoes is to start with high starch potatoes, such as Russets, that will help to thicken the sauce, use very fresh cream or half-and-half rather than whole or low-fat milk, and thicken the cream with flour or corn starch to make a sauce rather than topping the ingredients with cream straight from the container. You can layer the cheese with the potatoes and onions or melt it into the thickened sauce. Bake the casserole low and slow, 350 degrees F. for about 2 hours, covered for all but the last 15 minutes.