1cupMontepulciano or other medium-bodied, dry red wine
Put butter, oil, and sliced onions in a large pan. Cook over medium-high heat until onions just start sizzling, then reduce heat to medium low and cover with a lid. Cook, stirring regularly, until the onions are well wilted.
Remove lid. Continue cooking, stirring regularly, until onions begin to brown. When you notice them starting to stick add a splash of water and stir vigorously. This will help the sugars brown without scorching. Repeat until onion are fully caramelized, which will take an hour at least. DO NOT RUSH!
Add wine, vinegar, and salt, bring to a simmer, and cook, stirring, until the sauce thickens, about ten minutes.
Taste to check seasoning, then add pepper and more salt as needed. Serve with pork chops, chicken thighs, roasted squash, or just about anything else you can think of.
This makes quite a bit of agrodolce. It will keep for about a week in the fridge and for several months in the freezer.