Start by melting the butter along with the canola oil in a large cast iron or heavy-bottomed skillet over medium-high heat. The blend of these two fats will give your onions a rich flavor and help them caramelize beautifully.
Once the butter is melted and foamy, add the sliced yellow onions to the skillet. Sprinkle them with the kosher salt and the leaves from the fresh thyme. Stir gently to coat the onions with the fats and seasonings.
Cook the onions for about 5 minutes, stirring occasionally. You want them to start softening but not browning at this point. The key is to allow them to sweat and release their natural sugars.
Reduce the heat to medium-low, and cover the skillet with a lid. This will trap moisture and help the onions cook down evenly. Stir every 10 to 15 minutes, allowing them to develop that gorgeous golden color.
As the onions continue to cook, they will reduce in size and become sweeter and more jammy in texture. You’ll begin to notice a rich, nutty aroma filling your kitchen. It’s tempting to rush this process, but patience is key!
If you notice any browning or crisping at the bottom, don’t fret! Just add a splash of water or broth to deglaze the pan, scraping up any flavorful bits stuck to the bottom.
In the last 10 minutes of cooking, if you’re using it, stir in the white balsamic vinegar. This will add a nice tang to balance the sweetness of the onions.
Continue cooking for up to 1 hour, depending on how deep you want the flavor to be. The longer you cook them, the richer the taste. Once they are deeply caramelized, remove them from heat and enjoy immediately, or store them for later use.
