Here comes a classic from Venice. Shortly before Xmas I always make simple dishes mainly with fish&seafood. I recommend to buy mild white onions as they are sweeter than normal onions. Also take care that you cook the sliced onions on the lowest lowest (!) heat setting. Its totally forbidden to burn the onions.
Classic would be Bigoli pasta, but they are not easy to find. So use Spaghetti. Also Parmesan is a no-go, instead use Moglica fritta based on old breadcrumbs.
P.S. perfect was a STAUB cocotte for this dish. I strongly recommend it to use it also for other dishes roasting, frying etc. and not only for braising.
https://www.wineberserkers.com/content/?p=5689
Merry Xmas,
Martin