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I think you could totally make this a day or two ahead! (I made my agrodolce for Thanksgiving over the weekend...)
You should absolutely be able to make this ahead. I made Merrill's Tuscan Onion Confit (https://food52.com/recipes...) on Sunday and plan to serve it on Thanksgiving.
Deborah is a long-time speaker for the vegetable side of the plate and author of The New Vegetarian Cooking for Everyone.
I too think you can make your onions a day or two ahead. Just don't cook them all the way through so that when you rewarm them they won't get mushy!
June is a trusted source on General Cooking.
Shoot! I think you could have gotten by with making this a week ahead!