How to Caramelize Onions for Burgers (Sweet, Jammy, No Sugar)

To caramelize onions for burgers, slice them thin, cook them in butter and oil over medium heat until soft, then turn the heat down and cook them slowly, stirring now and then, for about 30 to 45 minutes until they're deep golden brown and jammy. You don't need sugar. Time and gentle heat bring out the onions' own sweetness.

Here's the full method, the mistakes that lead to bitter or soggy onions and how to make a batch ahead so you always have them ready.

What you need

• 2 large yellow onions (enough for about 4 burgers)
• 1 tablespoon butter and 1 tablespoon neutral oil
• ½ teaspoon kosher salt
• A few splashes of water, broth, wine or balsamic vinegar for deglazing
• A wide, heavy skillet, such as a cast iron skillet or stainless steel pan

Which onions are best?

• Yellow onions are the go-to. They have a good balance of sweetness and savory bite and hold up to long cooking.
• Sweet onions (like Vidalia) caramelize nicely but end up very sweet and soft. Great if you like a jammy topping.
• Red onions work too and turn a deep purple-brown with a slightly sharper flavor.
• White onions are sharper and a bit less sweet once cooked, but still fine.

Step by step

1. Slice evenly. Trim and halve the onions, then slice them thin, about ⅛ inch, from root to tip (pole to pole). Even slices cook at the same rate, and pole-to-pole slices hold their shape better.
2. Start over medium heat. Melt the butter with the oil in a wide skillet. Add the onions and salt and stir to coat. The salt draws out moisture and helps them soften.
3. Let them soften. Cook, stirring occasionally, for about 10 minutes until the onions are limp and translucent.
4. Turn it down. Lower the heat to medium-low. From here, it's slow and steady.
5. Stir every few minutes. Let the onions sit long enough to brown, then stir. They'll go from pale gold to amber to deep brown over 30 to 45 minutes total.
6. Deglaze as you go. When brown bits (fond) build up on the pan, add a tablespoon or two of water and scrape them back into the onions. That fond is concentrated flavor.
7. Finish. When the onions are soft, sticky and deep brown, taste and adjust the salt. A small splash of balsamic vinegar or Worcestershire at the end adds depth.

Common mistakes

• Too much heat. High heat scorches the outside before the sugars have time to develop, leaving you with bitter, unevenly browned onions.
• Crowding a small pan. Piled-up onions steam instead of browning. Use the widest pan you have.
• Stirring constantly. The onions need contact with the pan to brown. Stir every few minutes, not nonstop.
• Adding sugar. Onions have plenty of natural sugar. Added sugar can make them candy-sweet and more likely to burn.
• Calling it done too soon. Golden onions after 15 minutes are sautéed, not caramelized. Real caramelized onions take time.
• Using a nonstick pan. It works, but it doesn't build much fond, so you lose some of the deep, browned flavor that comes from deglazing.

Can you make them faster?

A little. Cooking over medium heat with more frequent deglazing can get you to deep brown in closer to 25 minutes, but you'll need to watch and stir more often. A pinch of baking soda speeds browning but makes the onions softer and mushier, so use it sparingly if at all. If you're short on time, the crispy, smashed-in onions of the Oklahoma fried onion burger are a faster way to get onion flavor onto a burger.

Make them ahead and store them safely

Caramelized onions are one of the best make-ahead burger toppings.

• Cool quickly. Spread the onions in a shallow container and refrigerate within 2 hours of cooking (1 hour if it's above 90°F), following USDA's guidance on leftovers.
• Fridge: store them covered and use within 3 to 4 days.
• Freezer: freeze in small portions (an ice cube tray or flat freezer bag works well). They stay safe frozen, and USDA notes leftovers keep their best quality for about 3 to 4 months.
• Reheat: warm them in a skillet with a splash of water until steaming hot, to 165°F.

How to add them to a burger

Warm the onions before serving so they don't chill the patty. Pile them on right after the cheese goes on, so the heat from the burger helps everything meld, or spread them on the bottom bun to catch juices. About 2 to 3 tablespoons per burger is a good amount.

Burgers to try them on

• The Mushroom Swiss Burger, where onions and mushrooms team up.
• A thick burger with blue cheese, Gruyère or sharp cheddar. See the best cheese for burgers.
• A rich dry-aged beef burger, where sweet onions balance the funk.
• Turkey burgers, which benefit from extra flavor and moisture.
• Stirred into mayo for a quick onion spread. More ideas in our best burger sauces guide.

For more ways to top your burger, browse the best burger toppings.

FAQ

How long does it take to caramelize onions?

Usually 30 to 45 minutes over medium to medium-low heat. Larger batches take longer because there's more moisture to cook off.

Should I add sugar to caramelized onions?

No. Onions contain plenty of natural sugar, and slow cooking brings it out. Added sugar mostly makes them burn faster.

Why are my caramelized onions bitter?

The heat was likely too high, scorching the onions or the fond in the pan. Lower the heat, stir more often and deglaze with a little water.

How long do caramelized onions last in the fridge?

Treat them like other cooked leftovers: refrigerate within 2 hours and use within 3 to 4 days, or freeze for longer storage.