The Absolute Best Way To Reheat Leftover Caramelized Onions

Some people think of a sandwich and default to a PBJ or a basic turkey and cheddar on white bread — both fine options, but certainly limited in creativity. True sandwich aficionados know that the sky is the limit when it comes to topping choices, and a good one is piled high with veggies, cheese, and condiments. The cherry on top for the ideal bread-encased bite? A scoop of perfectly sweet, slightly crisp caramelized onions. But doing this well takes a long time — as in, sometimes more than an hour of carefully watching and stirring them to perfection, and even longer if you want the perfect 10-hour slow cooker caramelized onions. Their one saving grace is that they keep pretty well in the fridge for up to a week, meaning that you can make one batch of several onions ahead of time and store them to use later. But can you reheat leftover caramelized onions and still make sure they retain their texture and flavor?

Eli Collins, chef at a.kitchen+bar, which has locations in Washington, D.C. and Philadelphia, told Chowhound in an exclusive conversation that it's totally possible and not too difficult with a little bit of patience. "Just warm [them] very gently over low heat so that [they] don't scorch," Collins recommended. And if you've got a larger amount of leftovers, he said you can even use a double boiler to warm them up as slowly and gently as possible. If they're looking a little mushy, Collins said adding a touch of water to your pan can help them reform a caramelized crust, and almost create the illusion that they've been cooked fresh.

Of course, part of the way to ensure your caramelized onions reheat well is to store them right. Eli Collins' secret method? "If you add a touch of vinegar (I prefer sherry vinegar) to the onions as they finish cooking, it will help add some preservative to them," he explained. Vinegar is an acidic substance, and its addition to your onions helps lower their pH level. As it turns out, acidic environments are not conducive to the growth of many bacteria and molds, so the vinegar slows down decomposition.

Whether you're prepping early to plan a successful dinner party or want to meal-prep a large serving of Gordon Ramsay's elevated grilled cheese sandwiches, Collins said cooking up a batch of caramelized onions even further in advance is totally doable if you freeze them. "They will thaw just fine without any change to flavor or texture," he told us. They can last like this in the freezer for several months, making it possible to devote some time to cooking down a whole batch of onions just once every season, slowly reheating small portions as the need arises.