Elevate the onion. No longer relegated to just a supporting role in sauces and stocks, the cipollini, or “little onion,” can stand on its own in a lovely little braise boosted by the porkiness of guanciale. The juices get all sticky and sweet, and if you don’t stop popping them into your mouth right out of the pan, dinner will never get on the table.
See Cook’s Note on how to peel onions.
In a pinch, you can make a lid to cover a braise. Tear off a piece of parchment that’s about the same size as your pan. Fold it in into sixteenths by folding it in half four times, just like when you were a kid making snowflakes out of construction paper. You should have a thinner triangle each time you fold. When you are done, point the tip of the triangle into the center of the pan and measure out to the edge. Trim the wider end of the triangle to make it fit in the pan. Snip off ½” from the point to make a hole in the center. Unfold and place on top of onions in the pan.
Melt butter and guanciale in a large skillet over medium heat and cook until just turning golden-brown, about 3 minutes. Remove any excess fat to keep 2 tablespoons in the pan. Add the onions in one layer and season with salt and pepper. Cook, flipping halfway, until nicely browned on both sides, about 8 minutes. Add the wine, port, vinegar, thyme sprigs and bay leaf and bring to a boil. Reduce heat to maintain a simmer, then partially cover and braise, turning at least once for even color, until onions are tender, 10 to 15 minutes. If necessary, continue to cook uncovered until the liquid reduces and glazes the onions. Discard bay leaf and thyme stems, sprinkle with thyme leaves and serve.
To peel onions, fill a soup pot halfway with water and bring it to a boil over high heat. Add onions, bring it back up to a boil and cook for a minute. Strain onions into a colander and rinse them with cold water to slightly cool. Trim ends and pop onions out of skin, removing the first layer along with the paper. If they are slippery, use a paper towel to hold the onions in one hand and your paring knife in the other. Place onions back in the colander and rinse one-by-one to remove membranes (otherwise they will glob up in the pan).