How to Caramelize Onions (and How Long It Takes)

What's happening in the pan

Onions are full of water and natural sugar. First the heat drives off the water and the onions wilt and turn soft. Then the sugars brown, and so do the onion's proteins, which is where the deep, savory, almost beefy flavor comes from. The brown crust that builds on the bottom of the pan is the most concentrated part of all, so you keep dissolving it back into the onions.

There's no shortcut around the water. It has to cook off before anything browns, and high heat only scorches the outside while the inside stays raw. That's why the clock matters more than the burner.

What you need

• Yellow onions, about 2 pounds (5 or 6 medium) for 1 1/2 cups cooked. They shrink by about three quarters, so start with more than seems sensible. Sweet onions work but taste flatter; red onions go a little purple-brown.
• A wide, heavy pan. Stainless steel or enameled cast iron lets you see and scrape the brown film. Nonstick won't build it at all.
• Butter for flavor, plus a little oil so it doesn't scorch. About 2 tablespoons total per 2 pounds.
• Salt, a cup of water near the stove, and a flat wooden spoon or spatula for scraping.

How to caramelize onions, step by step

1. Halve the onions through the root, peel, and slice them thin, about 1/8 inch, from root to tip. Slicing with the grain keeps the slices from falling apart into mush.
2. Melt the butter with the oil in the pan over medium heat. Add the onions and a big pinch of salt and stir to coat. The pan will look overfilled. It won't for long.
3. Cook, stirring every few minutes, until the onions are soft and wilted and start to turn golden, about 15 to 20 minutes. If they brown at the edges before softening, lower the heat a notch.
4. Turn the heat to medium-low. Keep stirring every 3 to 5 minutes, spreading them back out across the pan each time.
5. When a brown film builds on the bottom, add 2 tablespoons of water and scrape it up. The water sizzles off in a minute and the color goes back into the onions. Do this as often as the film comes back.
6. Keep going until they're the color you want (see the chart). Taste for salt. A splash of sherry, wine or vinegar at the end brightens them.

How long it takes, and what each stage is for

A faster way

If you have 20 to 25 minutes instead of an hour, start the onions over medium-high heat with a lid on for the first 10 minutes. The steam softens them quickly. Then uncover, lower the heat to medium, and deglaze with water every time the film builds, which will be often.

A tiny pinch of baking soda, about 1/8 teaspoon per 2 pounds, speeds browning noticeably because browning goes faster in a less acidic pan. Use it only for onions that will melt into a soup, dip or sauce. Any more than that, or too long in the pan, and they turn mushy with a faint soapy taste.

Common mistakes

• Cranking the heat. You'll get onions that are burnt at the edges and raw in the middle. Patience on medium-low beats a fire every time.
• Crowding a small pan, or spreading a few onions in a huge one. Too crowded and they steam for ages; too sparse and they burn. They should start about 2 inches deep and shrink down.
• Skipping the salt. It pulls water out early and seasons the whole batch. Add it at the start.
• Scraping the brown film into the trash. It's the best part. Deglaze it with a splash of water and stir it back in.
• Using a nonstick pan. Nothing sticks, so no film forms, and you lose the deepest flavor.
• Adding sugar. Onions have plenty of their own. Sugar mostly makes them burn faster.

Make a big batch and store it

Since it takes the same hour either way, cook as many as your widest pot will hold. Caramelized onions keep a week in a covered container in the fridge. To freeze, spoon them into an ice cube tray, freeze until solid, and pop the cubes into a bag; they keep for 3 months and thaw in minutes in a warm pan. One cube stirred into a pan sauce, beef stroganoff with browned mushrooms or a pot of beans earns its keep.

You can also make them in a slow cooker: 3 to 4 pounds of sliced onions with butter and salt, on low with the lid cracked, for about 10 hours, stirring once or twice. They come out softer and less deeply browned than the stovetop kind, so finish them in a skillet for 10 minutes if you want more color.