Preparation

• Heat the oven to 400°F. Set a large ovenproof skillet (or a flameproof roasting dish) over medium-high heat for 30 seconds. Put in the olive oil and butter. When the butter foams or browns slightly, add the onions, cut side up. Cook until the onions brown slightly, about 3 minutes. Flip them so the cut sides face down and put the pan in the oven. Roast until the onions are very tender and dark golden brown, 50 to 60 minutes. Remove from the oven and sprinkle with the sherry vinegar, salt, thyme, and sugar. Let cool to room temperature and then slice into thin strips; pour any juices from the pan over them and mix them in.

Make Ahead Tips

You can make this recipe ahead of time and store it in the refrigerator for up to three days.