Slow Cooker French Onion Soup

This Crockpot French Onion Soup is easy to make, packed with flavor, and surprisingly light and healthy. It's delicious on its own or topped with toasted bread and melted cheese.

It’s funny how you forget about the food you really love sometimes. This is what happened to me with French Onion Soup until a few days ago.

I was out for lunch with a friend and we had just ordered our own food when the irresistible aroma hit me. I turned toward it to find that all four people at the table next to us were just diving into their French Onion soups. There it was, flooding my nostrils with its deep and rich onion flavor, crunchy bread, and melty cheese. I immediately knew what I was making for dinner the next day.

However, with a busy day in front of me, I knew I wanted to make this French Onion Soup in the slow cooker. This eliminates the need to stand at the stove for hours making caramelized onions in a skillet. Instead, the slow cooker does that hard work for you.

Once the onions are caramelized, all it needs is some time in the slow cooker with chicken (or beef broth) anr fresh thyme for the flavors to get rich and delicious.

Top it with some baguette slices and gruyere cheese, pop it under the broiler, and you have restaurant-quality onion soup at home.

Love making soup in the slow cooker? Don't miss this delicious Taco Soup, Lasagna Soup, and Slow Cooker Lentil Soup.

Ingredients and Substitutions

Here is everything you need to make delicious French Onion soup in the crockpot.

• Onions: Onions are the most important part of this soup and to get that rich, deep oniony flavor, it is critical that the onions get nicely caramelized. Any onions or combination of onions can be used but we recommend sweet onions, like Vidalia onions, for the best flavor. White onions, red onions, and yellow onions can all be used.
• Butter: Butter adds richness to the soup and adds more flavor than olive oil. With that said, you could use olive oil instead or another neutral oil.
• Worcestershire sauce: Worcestershire sauce adds that deep, umami flavor to the onion soup. If you don't have it, use soy sauce instead.
• Balsamic vinegar: This adds a touch of acid to the soup and also brings out the sweetness of the onions. Swap in apple cider vinegar if needed. Many recipes also use cognac, Armagnac, or brandy instead of vinegar.

• Garlic: French onion soup traditionally doesn't contain garlic but we love adding some to the soup to deepen the flavor.
• Brown sugar: Sugar helps the onions to brown and caramelize. This is especially helpful since we are caramelizing the onions quickly in the slow cooker. It can be left out but the onions won't be quite as browned and sweet.
• Flour: Flour is used to slightly thicken the soup. Some purists believe French onion soup should be thinner without any thickener or flour. This is up to personal preference.
• Broth: Traditional French onion soup almost always uses beef broth. We enjoy a lighter flavor, so normally I make it with chicken or vegetable broth instead. Any broth can be used.

• Fresh thyme: Onion soup almost always use fresh thyme sprigs. Fresh will give the best flavor but dried can be used if that's all you have. You can also add 1-2 bay leaves to the broth.
• Kosher salt and black pepper: Make sure the soup is really well seasoned. If it tastes bland at all, it probably needs more salt.
• Bread and cheese: Finish the soup under the broiler with some toasted French bread and melted gruyere cheese for the most traditional option. Mozzarella cheese, Swiss cheese, and Parmesan cheese are also used.

Best Cheese for French Onion Soup

Traditionally, this soup is served in a cup, ramekin, or soup bowl (or a mini cast-iron Dutch oven would be fun too) with a big slice of French bread on top (to serve as a giant crouton). The bread is then covered in aged gruyere and broiled until it's nice and toasty and bubbly.

If you don't have gruyere, don't worry. Swiss cheese is perfectly suitable as a substitute and it's what I always tend to use. Other cheeses you could use include provolone, parmesan, Jarlsberg, Beaufort, or Comte.

How to Make French Onion Soup from Scratch

The key to making French onion soup from scratch is cooking down the onions until they get browned, caramelized, and super soft. This is what gives the soup its deep and rich onion flavor.

Whether you are making it in a slow cooker, stovetop, or Instant Pot - you always need to caramelize the onions. This is where the flavor comes from.

If you are truly making the soup from scratch, you will also want to use homemade beef broth or chicken broth. However most times I just use high-quality chicken or beef stock from the store.

Once you have done that, the rest is easy. You just need broth, traditionally beef broth is used, some herbs like thyme, and then actual time. Cooking the soup for a long time is another key for French Onion soup, it develops the flavors.

Once the soup is finished, it's time to prepare the toast and melted cheese. Use a good quality French baguette and cut it into slices. Toast the slices in the oven on a baking sheet. Once they are lightly golden brown, remove them from the oven.

Add the bread slices to the soup, ensuring they are in oven-safe bowls. Sprinkle with gruyere cheese and return to the oven. Remove once the cheese is bubbling and golden brown.

What Makes This Slow Cooker French Onion Soup Healthy?

Don't let the bread and cheese fool you — this soup isn't as caloric as you might think. In fact, one cup (without the bread and cheese) has only 100 calories.

In addition:

• Onions are a great source of antioxidants and they also contain folic acid, fiber, and vitamin B which is responsible for healthy new cell creation.
• Butter (just a little) provides fat that your body needs to keep your systems running smoothly and is also used as brain food.
• Worcestershire sauce and balsamic vinegar are similar in their nutritive properties, in that they both help with digestion, blood circulation, and enhanced immunity effects, and have both been noted to improve digestion, nervous system function, and fight against free radicals.
• Garlic is known for its immune system-boosting properties. It also is used as an antifungal and antiviral and also has antioxidant properties. It's also heralded as a heart helper and can lower your cholesterol.
• Spices such as salt, pepper, and thyme add flavor and inflammation-boosting properties without adding calories or fat.
• Vegetable broth (or chicken or beef) is known to help your hair, nails, and skin remain healthy and vibrant. Animal broths are also good for your muscles, connective tissues, and joints.

Side Dishes to Serve With Crockpot French Onion Soup

If you ask me, no soup is complete without a side salad or sandwich. To that end, some things I'd serve with this soup include:

• A simple spring mix salad works well here too, get a nice spring mix and add in shredded carrots, thinly sliced bell peppers, some cucumbers, and cherry tomatoes, sliced in half longways. Add feta or goat cheese crumbles or some slivered almonds and your favorite light dressing.
• This Israeli Chopped Salad is a family favorite and can be made a day or two in advance and brought back out and served with just about anything you can think of — including piping hot soup!
• This Easy Lentil Salad is another one that's easy to put on repeat and eat, eat, eat. Plus it adds protein to the meal.
• If you want the soup to be your main focus, you can also serve a mini charcuterie board on the side with some more thinly sliced bread and an assortment of cheeses, fruit, and pickled veggies.

Storage and Freezing

French Onion Soup only gets better after a night in the fridge for the flavors to depend and combine. It also freezer really well.

• Fridge: Let the soup cool completely and store in the fridge for 4-5 days. Reheat on the stove or in the microwave.
• Freezer: Once the soup has fully cooled, it can be kept in the freezer for up to 3 months. Let defrost overnight in the fridge and then reheat. Or add it to a pot (or the slow cooker) from frozen and let it slowly reheat.