Caramelized onions add a whole lot of flavor with very few ingredients and even less effort, so long as you know the secrets. Here's how to caramelize onions.
Melt the butter with the canola oil in a large cast iron or heavy bottomed skillet over medium high heat.
Add the onions and sprinkle with the kosher salt and fresh thyme leaves. Cook the onions for 5 minutes or until they start to soften, stirring so they don't crisp or brown in spots.
Reduce the heat to medium low, cover with a lid, and continue to cook, stirring the onions every 10-15 minutes or so. The onions will reduce and become more golden and jammy as they cook, giving off a rich nutty aroma. If the onions look like they're beginning to burn or crisp, stir in a splash of water or broth and scrape the caramelized fond from the bottom of the pan for the onions to absorb. Add the vinegar if desired in the last 10 minutes of cooking.
Continue to cook up to 1 hour or so for a deep, rich caramelized flavor. Serve immediately or refrigerate for up to one week.
Caramelized onions take time for the flavor to develop but not a whole lot of hand-holding. Allow the onions to cook undisturbed in between stirring times for the caramelization to happen naturally.