Last night I made some scalloped au gratin potatoes, with a layer of thin sliced ham and a little lacinto kale for color. I cooked some onions in a 1/4 cup of butter, then stirred in a tablespoon of flour, followed after cooking it in by 1 3/4 cups of milk and a cup and a half of cheese and brought it to a boil then simmered a minute or so until it thickened. This was poured over the potato assemblage and baked at 325 for 1 hour 20 minutes, then a bread crumb cheese topping followed by another 20 minutes to brown.
The dish came out of the oven with a pool of clear liquid around the perimeter of the spuds. When removing a serving the presence of what looked like curds were in the liquid. I assume that this is what is known as a "broken sauce". Can anyone tell me how to this happened and what I can do to prevent another occurrence?
Thanks for any insight!