For Hands-off Caramelized Onions, Start in the Microwave

I love caramelized onions. They can transform a simple burger into something gourmet, instantly upgrade a pasta or legume dish, and add some sweet-savory depth to just about anything. If I am honest, I've always hated making them. Standing at the stove, stirring every so often for what feels like forever, is much too time-consuming for me.

When I came across Sarah DiGregorio's big-batch method, it sounded too good to be true. It uses the microwave to jump-start the process, then moves on to some hands-off slow-cooker time. Since trying this method a few years ago, I've never gone back to my old ways!

How To Make (Almost) Hands-Free Caramelized Onions

Adapted from a Cook's Illustrated method, DiGregorio first uses the microwave to remove most of the moisture. The slow cooker, then, works its magic while I do (literally) anything else. The best part about it is that it's almost entirely hands-off. After the initial prep time, the slow cooker does the bulk of the work.

I always start with the recommended four to five pounds of sliced yellow or red onions. It sounds like a lot, but the onions shrink dramatically after cooking. Slicing the onions is the most effort I put in during the entire process. I pile the onions into a large microwave-safe bowl, season them generously with kosher salt and a tablespoon of oil.

I then microwave everything on high for 15 minutes. During those 15 minutes, I stir the onions every five minutes. The microwave will heat them from the inside out, breaking down the cell walls and quickly releasing all the liquid. The first time I did it, I was surprised by how much the onions collapsed and how much water was released.

Before transferring the onions to my slow cooker, I drain off all the liquid—this is key! Then I add in more flavor by stirring in half a stick of unsalted butter, a sprig of fresh thyme, and two more teaspoons of salt.

The rest of the process is beyond easy. I just put the lid on the slow cooker and set it to high for nine hours, stirring only twice. During this long, gentle cook time, the onions' natural sugars caramelize slowly and evenly, developing a deep, complex flavor.

In the final 30 minutes, I stir in two teaspoons of balsamic vinegar and prop the lid slightly open to allow any remaining moisture to evaporate. The result? Thick, jammy, and intensely flavorful onions.

Why It Works

Onions are about 90% water, and water, as you might have guessed, is the enemy of caramelization because it makes them steam instead of brown. The microwave rapidly removes most of that moisture by exciting the water molecules inside the onions, causing them to heat up and evaporate quickly. The dehydration process, which typically takes 30 to 45 minutes of stovetop stirring, can happen in just 15 minutes in the microwave.

The slow cooker keeps everything at a low, steady heat. It's the perfect environment for encouraging the Maillard reaction, which slowly turns the onions deep golden brown. 

Because the onions start a little dehydrated from the microwave, they can get straight to browning. Over the next several hours, the natural sugars caramelize and deepen, turning them sweet, savory, and a little nutty.

Tips for Making the Best Caramelized Onions

• Salt isn't just for seasoning: Make sure you add that initial pinch to the onions before the microwave step. Salt helps draw moisture out of the cell walls, making the dehydration process even more efficient.
• Acid is important: The two teaspoons of balsamic vinegar at the end, adds a hit of acidity to balance the sweetness and make the flavor more complex.
• Freeze in portions: I use an ice cube tray or small containers so I can grab just what I need to add a quick and easy flavor boost. In the freezer, they keep beautifully for up to six months.