First you need to caramelize the onions. This requires sweating the onion: place one sliced medium red onion in a covered pan over medium heat with 1 tablespoon olive oil, 1 tablespoon granulated sugar and 1/2 teaspoon Kosher salt. Sweating releases water and the sugar and salt expedite the process as well as adding flavor. Once the onion is softened, about 5 minutes, remove the cover and raise the heat. The natural sugar in the onion brown the bottom of the pan, so have a cup of water nearby. Once the fond (the brown sugars) is apparent on the bottom of the pan, pour in a dash of water and scrape the bottom of the pan with a wooden spoon. The fond dissolves and colors and flavors the onion Do this three of four times until the onion is very soft. Off heat, stir in 1 tablespoon of red-wine vinegar, which adds tartness to compliment the sweetness from the sugar and onions. Serve immediately or store and reheat for later.