This roasting method works with other vegetables besides onions: radicchio, endive, and eggplant are favorites.
My favorite way to have a supply of cooked onions at the ready is to roast them in their own skins. Medium-sized onions work best—small ones can become über-charred in a very hot oven, and it may be difficult to roast large ones all the way through without carbonizing the outer layers before the heart of the onion is tender. Rake some coals into an even layer just inside the door and toss a few onions onto the layer. You can also reach in with gloved hands or tongs to place them on the coals. The skins will immediately start to burn into a shell, protecting the onion as it starts to roast in its own juices. Turn the onions over a few times. You may need to pull them forward to a hot part of the oven that’s free of coals so they continue to cook to the center without increasing the amount of charred exterior. They are done when a skewer penetrates to the center with no resistance. Take them out a little earlier if you want some onions that are slightly crisp in the middle and still have a hint of fresh onion flavor.
From the Wood-Fired Oven by Richard Miscovich
Remove the onions from the oven, place in a covered container, and let them cool in their skins so the smoky, charred flavor seeps into the flesh of the onions. Once they’re cool, peel off the skin and you’ll have an orb of cooked onion. Don’t rinse off the small, flavorful bits of charred skin that stay on the onion. Rinsing the onion will wash away some of the juice released during roasting.
Onions can also be roasted at a lower temperature and without the fire, although the smoky flavor won’t be imparted.
You can simply stash roasted onions in your fridge. Make a simple side dish by quartering them or slicing them into rings and dressing with olive oil, red wine vinegar, and salt and pepper.
This recipe is adapted from Richard Miscovich’s book From the Wood-Fired Oven: New and Traditional Techniques for Cooking and Baking with Fire (Chelsea Green, 2013) and is printed with permission from the publisher.