Step 1

Heat your oven to 425 degrees F.

Step 2

Bring a large saucepan of water to a boil and add about a tablespoon of salt for each quart of water.

Step 3

Add the unpeeled onions, and boil for 2 to 3 minutes. Drain the onions in a colander and rinse with cold water to stop the cooking. Rub the onions in a dry dish towel to remove the peel.

Step 4

In an oven proof skillet, heat the olive oil, sugar, and 1/4 cup of the sherry vinegar and then add the onions, sprinkle with salt and pepper, and bring the ingredients to a simmer. Turn off the heat, cover the pan, and transfer it to the oven. Roast for 20 minutes.

Step 5

Remove the lid and continue to roast for 10 more minutes, tossing the onions in the liquid every few minutes to glaze.

Step 6

Remove the pan from the oven and heat it on the stovetop over medium high heat and season again with salt and pepper. Cook, tossing the onions gently until they are lightly glazed, 4 to 5 minutes. Add two tablespoons of the sherry vinegar, two tablespoons of water, and cook over medium heat for 2 to 3 more minutes, gently tossing until the onions are deeply glazed and the sauce is thickened. Turn off the heat, sprinkle with the thyme leaves, a little more salt, and gently toss.