Properly caramelized onions—deep, sweet, and soft—cannot be rushed. You can cheat with a little sugar and baking soda, but if you’re a purist, you’re going to be hanging out at that pan for at least 45 minutes, probably more. While you’re there, you might as well make a lot, and freeze the leftovers.
No one in the history of cooking has every made too many caramelized onions. It’s much more likely that you’ve watched in dismay as a jumbo-ass onion cooked down to a mere half cup of sweet, sticky slices or bits, then thought “Dang it. I should have made more.” And you should have made more. You should always make more. Every time you find yourself making caramelized onions—for dips, or eggs, or burgers—you should at least double, if not quadruple, the amount. Aim for an absurd amount of caramelized onions, then freeze any that you don’t manage to consume in one sitting.
Caramelized onions freeze beautifully, and you should take advantage of that. Imagine a life where you can add deep, sweet umami to soups, stews, braises, burgers, and anything else with a quick trip to the freezer. Don’t you want that for yourself? Don’t you want access to easy flavor?
How to freeze carmelized onions
There are a few ways to freeze caramelized onions. Some people use an ice cube tray, but an ice cube of onions is a paltry amount. A muffin tin is far more appropriate: Just line the tin with plastic wrap, smoosh the onions down in there, then cover with another layer of plastic wrap and pop the whole tray in the freezer. Once they have set, remove the frozen onion pucks from the tin and store them in a freezer bag (in the freezer).
If you don’t have a muffin tin (or are currently using it to bake some muffins), you can also freeze onions directly in a freezer bag. Fill the bag, flattening it as you go and pushing out as much air as possible. This helps prevent ice crystal formation, and freezing your onions in a thin sheet, rather than a block, makes it easier to break off as much or as little as you need. (Want to make it even easier? Use a chopstick or pencil to create portion lines.)
When you’re ready to use your frozen onions, chuck the frozen pucks or sheets directly into a bubbling liquid, or melt them in a nonstick pan over low heat until they are warmed through, then spread on burgers or mix into dips. This is likely to increase your caramelized onion uptake by a whole lot, something I think you very much deserve.