This is really old-fashioned and a dream of a dish. I used to make sauce soubise (made from béchamel and puréed onions) from time to time for roast lamb, but never thought of putting an onion sauce with chicken. Then I read about Breton onion sauce (similar to sauce soubise) in one of my favourite cookbooks, Jenny Baker’s Cuisine Grandmère (it’s out of print but try to track down a copy, it’s a gem). She suggested serving it with roast chicken. My Breton sauce is based on her recipe. It’s basically a cream-enriched béchamel mixed with a soft, melting mass of cooked onions and a good slug of Calvados.
It’s best to back-time the béchamel to be ready just when the chicken has rested. You can do the onion component in advance, but reheating béchamel is always a bit of a pain.