When I saw an article in the food section of our newspaper last week saying you could make caramelized onions and, thus, french onion soup in a slow cooker, I was skeptical. Given how much moisture tends to collect in a slow cooker, I just didn’t think you could get the beautiful caramel color and slightly sweet flavor that browning onions on a stove top could achieve. It turns out you can make a very rich and deep flavored french onion soup from the slow cooker BUT it does require a little know how. After cooking 12 pounds of onions this week (that’s right – 12 pounds), I’ve gathered a few tips and insights to share with you.
For my first batch of onions, I cooked 3 pounds of onions on low heat in my slow cooker for almost 12 hours as instructed by the original recipe. The result was some soft cooked onions with lots of liquid and very little caramelization. The onions tasted fine but they didn’t have the beautiful color or deep flavor I was hoping for.
I decided to try another batch with cooking the same amount of onions but on high heat. The temperature change made all the difference. The extra heat prevented too much moisture from building up and allowed the onions to actually brown. After only 8 hours, the onions were perfectly caramelized. I had also stirred everything on occasion during the 8 hours to promote even browning.
I was then curious if I could cook more onions and achieve the same effect. The answer was no. After trying to cook 6 pounds of onions on high for 12 hours, I only got soft cooked onions again. Even allowing the onions to cook uncovered didn’t help. Apparently, 6 pounds of onions released far too much liquid that couldn’t be cooked off to allow for browning. Three pounds of onions on high was the winning combination for me. I wasn’t crazy enough to try more batches with 4 and 5 pounds. I should note that I use a 6 quart capacity slow cooker with an oval shaped insert. I think the larger pan allows the onions to spread more which helps with the caramelization. Of course, the caveat is that different slow cookers cook at different temperatures. One brand’s low temperature may be another’s high temperature setting. My slow cooker isn’t particularly fancy. It’s just a mid-level Hamilton Beach programable slow-cooker.
You might notice the lack of melted, stretchy cheese on top of the soup. I know that is the classic way to serve french onion soup and it makes for great food porn shots but the truth is – I don’t like melted cheese in my onion soup. I love melty gooey cheese on many other things but I find it to be too hard to eat in a soup like this. You inevitably end up with one large glob of cheese in a single bite and no cheese for the remaining soup. Instead, I prefer to serve cheese crostini along side the soup. You can dip the crostini in the soup and enjoy the combination of flavors for the entire bowl.
- 3 pounds yellow onions
- 4 tablespoons butter, melted
- 1/2 teaspoon kosher salt
- 1/4 cup dry sherry
- 2 cups chicken broth, heated
- 2 cups beef broth, heated
- Salt and pepper to taste
- 8 diagonally cut (1/4-inch-thick) baguette slices
- Olive Oil
- 1/4 cup grated Gruyére cheese or more to taste
- Dried thyme for garnish, optional
- Cut the onions in half lengthwise (stem to root), remove the onion skins and discard. Slice the onion halves into 1/8 to 1/4 inch thick slices, discarding the root ends. Place all the onion slices into the insert of a 6-quart slow cooker. Add the melted butter and salt; toss to coat evenly. Cover the slow cooker and cook on high heat for 8-9 hours, stirring occasionally. At this point, the onions should be a deep brown color and there should be only a little bit of liquid in the pan.
- To make the soup, turn the slow cooker to high heat. Add the sherry and stir until most of the liquid is absorbed. Add the chicken and beef broths and stir to combine. Let soup simmer for 15-30 minutes and is heated through. Taste soup and season with salt and pepper to taste.
- While the soup is simmering, heat oven to 400 degrees. Brush olive oil on baguette slices and place slices on a baking sheet in a single layer. Bake until bread is crisp, and just golden at the edges, 6-10 minutes. Sprinkle cheese on the baguette slices and return pan to the oven. Continue baking until cheese is melted. Sprinkle with dried thyme, if desired.
- Ladle hot soup into serving bowls and serve with cheesy baguette slices.
Try to use chicken and beef broths with the least amount of sodium as possible.
This recipe makes a fairly thick soup. Feel free to add more broth if you want a thinner soup.
Recipe for caramelized onions adapted from the Houston Chronicle.
Recipe for soup adapted from Cook’s Illustrated.