I came across this in the context of low-carb chicken pie filling, in other words without a flour/roux base. Is there a name for a sauce that starts with low & slow-cooked onions?
Asking for a friend ... really ;-)
A soubise is a classic french onion-based sauce. Really it is just slow cooked onions with a bechamel added. Of course, bechamel has flour in it to thicken. However, I have seen recipes where the bechamel is replaced with heavy cream. For example, here, onion, cream, and vadouvan spice are blended together to make a flour-free soubise. It would make a terrific pot pie filling!