Caramelized onions are not synonymous with grilled onions. Both have their place and grilled onions can be delicious, but grilling is about high heat and caramelization happens slowly over low heat.
Caramelization is a chemical change that makes carbohydrates (e.g. sugars) turn brown when heated to temperatures of 310 F or higher. The best onions for caramelizing are the so-called "sweet" onion varieties:
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Heat Oil in a Nonstick Pan
Let the oil get hot for a moment. A general rule of thumb is to use 2 tablespoons of oil for every three onions. If you like, you can use a mixture of oil and butter. Clarified butter is best, since it won't burn as easily as ordinary butter. As long as you don't let things get too hot, your butter won't burn.
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Let the Onions Turn Translucent
As you cook them, the onions will start to soften and turn translucent. The reason sweet onions are used is that they have a higher sugar content and it's the sugar that's actually turning brown. Ordinary yellow onions have some–but not as much–sugar in them and you can certainly caramelize them as well.Continue to 5 of 7 below.
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The Caramelized Onions
Here they are—a rich, golden brown hue and bursting with smoky, sweet onion flavor. Need an excuse to throw a few burgers on the grill? Delicious caramelized onions like this are all the reason you need. Also, try this classic French onion soup.