Sounds great - would love to make ahead. Any reason why it couldn't be frozen?

I often freeze gravy but find that the flavors can dull just a bit - so be sure to check for salt, and don't hesitate to add a light splash of red wine or good cider vinegar if you think the flavors could use a bit of brightening, right before serving. (For a gravy like this, I'd probably do that anyway, even if not freezing, given the intense sweetness of caramelized onions. A dollop of a high quality sharp mustard would not be amiss, either.). ;o)

Not at all, feel free to freeze it! Thaw it in the fridge overnight, then simmer it for a few minutes before serving :)