These onions agrodolce, or sweet-and-sour onions, are a perfect example of how I cook in Italy. I buy my produce from the local town and season it with garden herbs and wild fennel from the hillsides. This creates an enticing and organic dish.
Preheat the oven to 400 degrees F. Cover the bottom of a medium, heavy-bottomed baking dish with the olive oil, balsamic vinegar, sugar, and fennel fronds. Season with salt.
Cut the onions in quarters lengthwise. Trim the root ends of the onions, still leaving them intact. Lay the onions cut side down in the baking dish in concentric circles, fitting them closely together. Nestle a fresh bay leaf between every few onion halves and top with the sage. Drizzle with a bit more olive oil and a few drops of balsamic vinegar. Season with salt.
Cover the pan with heavy-duty, restaurant-grade plastic wrap, which won't melt in the oven, and then aluminum foil. (Conventional plastic wrap will melt in the oven, so if you do not have restaurant-grade plastic wrap, skip it.) Put the onions in the oven until they're softened, about 40 minutes.
To check for doneness, remove the baking dish from the oven and carefully lift up one segment of the foil and plastic if you're using it, taking care not to burn yourself from the steam that arises from the pan; use a small sharp knife to check to see if the onions are tender. Otherwise, return the foil and plastic and return the onions to the oven for another 10 to 15 minutes, or until they're tender.