I have a problem that I need some resolution with.
I am making a potatoes au gratin recipe that calls for making a blonde roux and allowing it to cool to room temperature, and then add the blonde roux into a boiling chicken stock (tones chicken base + water), after which heavy cream is added to cut the stock down a bit. The consistency is sort of a thicker napé that congelates upon refrigeration (this helps to keep the layers of potatoes together and the au gratin potatoes can then be cut into squares and served).
The problem that I am having is this:
Once the consistency is achieved after adding the roux slowly to the boiling stock (the stock boils for about 5-10 mins), I reduce the heat just enough to keep it at a simmer (very soft boil) with a few slow bubbles coming up and then attempt to let the sauce simmer for at least 20 minutes, but the roux seems to separate (break down) in the sauce after about 5-10 minutes, releasing all the fat into the sauce and thickening the center portion of the sauce. I'm assuming the fat is basically separating from the flour. Is there any way around this? Why is the roux breaking down in my sauce while it is simmering, when supposedly it should be stable enough to allow a sauce to simmer for 20-30 mins without having the butter leak out?
For the roux I am using Gold Metal All-Purpose Bleached, Enriched Flour and Salted Tillamook butter. For the proportions of the roux I am using 1 cup butter to 1 heaping cup All-Purpose Flour, whisking until the consistency becomes a slightly runny paste with a buttery sheen.
Please let me know your thoughts on why my roux is breaking down during the simmering process, and if there is any way to avoid it. I am simmering it at a medium to medium-high heat, just enough to form a few bubbles that rise to the surface.