The Reason You Shouldn't Slice Onions Too Thin When Caramelizing Them
Caramelized onions are a key ingredient in many recipes. You can't have French onion soup or pissaladière without them, and any time you add them to burgers or dip, it just takes the taste to another level. Onions cooked normally, as well as onion powder, are both great at adding a distinct pungency to any dish, but caramelized onions have a different flavor profile.
As Simply Recipes explains, the caramelization process is the only way to bring out the natural sweetness in the onion. The heat draws out the sugars as the onions cook, and these sugars begin to caramelize, giving the onions a rich brown color. What results is a uniquely sweet and savory flavor that goes well with many recipes. It's common knowledge that cooking onions low and slow is necessary to successfully caramelize them, but what you may not know is that the width of the actual slices is equally as important.
It's harder to caramelize thinly sliced onions
While you might assume that cutting your onions thin will cut the cooking time in half while allowing each layer to caramelize, it actually does the opposite. Rather, according to The Kitchn, the onions will char before the sugar even has the chance to get out, and all that delicious flavor that usually develops in the process will be null and void. So if you want to ensure every layer of your onions gets caramelized, the best way to go about it is to cut your onions into even-sized pieces.
Food & Wine recommends cutting your onions into either 1/2-inch or 1/4-inch thick segments. The thicker the pieces, the longer they'll take to cook, but as the publication states, they'll eventually shrink down to a third of their original size. Per Serious Eats, you can caramelize any variety of onions as they all contain sugar. Still, no matter what type you use, just be sure to keep the slices relatively thick, so they cook rather than burn.