With a caramelized onion and beef stock base, this slow cooker French onion soup is deeply savory, shockingly simple, and 100% gourmet.
French onion soup is an 18th century treasure. Stemming from the plentiful and economical onion, this classic soup marries a sweet, flavorful broth with an iconic topping of toasted baguette and gooey cheese.
The hardest part of the recipe might be slicing all of the onions, but the crock pot will take it from there. It’s a minimal effort show-stopper, which is a big deal for busy moms.
This flavorful soup is richly developed, with the sweetness of the sliced onions and the salty, beefy broth.
Serve it as the main course with a hearty topping of sliced baguette and melty cheese, with extra slices of baguette to last you the whole bowl.
Pair with a fresh, green salad and lemon vinaigrette or a side of deviled eggs, or make it a hearty side dish alongside a whole roasted chicken.
If you’re interested in baking homemade bread for the topping, I’d suggest my sourdough baguette recipe, an easy sourdough Italian bread, or this sourdough French bread.
This recipe is a great way to fill your home with delightful aromas and flavors while having your slow cooker handle the heavy lifting.
Why You’ll Love This Recipe
Delicious, rich flavor – With the sticky-sweet perfection of caramelized onions and a broth enhanced with red wine and fresh herbs, the depth of flavor in this simple soup is pretty incredible.
Low maintenance slow cooking – From caramelizing raw onions to developing the rich broth, you’re essentially just stirring, seasoning, and assembling.
Elegant comfort food – French onion soup is a top-tier cozy dish. Served in bowls with a topping of crusty bread and melty cheese, you’re going to be sharing this delicious recipe for a long time.
Ingredients
Onions – I think the best onions for caramelizing are sweet varieties like Vidalia and Walla Walla.
Red wine – Wine adds a lot of great flavor. Use one that you enjoy drinking!
Beef broth – You can use store-bought or homemade beef broth, or try bone broth for a more intense beef flavor.
Baguette – Grab a loaf or two from the grocery store, or bake your own sourdough baguettes for maximum flavor.
Gruyère – Gruyère is an excellent melting cheese with bold flavor and a smooth texture. You could use Swiss, provolone, or mozzarella if that’s what you have on hand.
A full ingredient list with exact amounts can be found in the recipe card below.
Tools You May Need
Slow cooker
How to Make Slow Cooker French Onion Soup
Step 1: Pour two tablespoons of melted butter into the bottom of a slow cooker. Add 3 pounds of thinly sliced sweet onions, stirring to coat evenly in the butter.
Step 2: Cover and cook on high for 5 to 6 hours, stirring occasionally, until the onions are deeply caramelized and golden brown.
Step 3: Add beef broth, red wine, fresh thyme, and bay leaves. Stir well, cover, and cook on high for 30 to 45 minutes.
Step 4: In a small bowl, whisk 2 tablespoons of all-purpose flour with about ½ cup of hot broth from the crockpot until smooth. Stir the flour mixture back into the soup to slightly thicken.
Step 5: Add ½ teaspoon of balsamic vinegar for acidity. Taste and season with salt and freshly ground black pepper. The soup should be well-seasoned and slightly salty.
Step 6: Preheat the broiler. Slice the French baguette into ½-inch thick slices and place on a parchment-lined baking sheet. Brush both sides with olive oil. Broil on the top rack for 2 to 3 minutes per side, or until golden brown — watch closely to avoid burning.
Step 7: Remove the bay leaves and ladle hot soup into oven-safe bowls or crocks. Top each bowl with 2 to 3 toasted baguette slices, then cover generously with grated Gruyère cheese.
Step 8: Place bowls on the baking sheet and broil for 1 to 2 minutes or until the cheese is melted and bubbly. Carefully remove from the oven (the bowls will be hot!) and serve immediately with a few sprigs of fresh thyme on top for garnish.
Tips
- Slice your onions very thinly.
- Toast extra baguette slices to add more at the bottom of your bowls.
- Stir the onions a few times while caramelizing. While the crockpot is low-maintenance, giving the onions a quick stir every 1 to 2 hours helps prevent burning around the edges and promotes even browning.
- Use a wine you’d enjoy drinking.
- Use sweet onions for the best caramelization. Varieties like Vidalia or Walla Walla give a mellow, rich flavor without bitterness.
- You can caramelize the onions a day in advance and refrigerate them—just add broth and finish the next day.
Recipe FAQs
Leftover soup (without the bread and cheese) keeps well in the fridge for 4–5 days. Store in a saucepan with a lid, or a glass airtight container.
Gruyère is classic, but Swiss, provolone, and mozzarella work really well, too.
No, you can toast or broil the bread and cheese on a baking sheet instead. Just add it right on top of each bowl of soup!
Just add more beef broth and a splash more balsamic vinegar or even Worcestershire sauce.
Sweet and yellow onion varieties caramelize well and are the most traditional. White onions have a more subtle flavor, while red onions have a more intense flavor and will affect the color of the soup.
More Soup Recipes from the Farmhouse
- Roasted Pumpkin Soup
- White Chicken Chili
- Roasted Red Pepper & Tomato Soup
- Homestyle Chicken & Noodle Soup
- Easy Cheeseburger Soup
If you try this recipe and love it, I would love it if you could come back and give it 5 stars! Tag me on Instagram @farmhouseonboone.
Slow Cooker French Onion Soup
Equipment
- 1 Slow Cooker
Ingredients
- 2 tablespoons butter
- 3 pounds of sweet onions
- ¾ cup red wine, like merlot
- 6 cups beef broth
- ½ tablespoon fresh thyme leaves, or 1 tsp dried
- 3 bay leaves
- 2 tablespoons all-purpose flour
- ½ teaspoon balsamic vinegar
For the topping
- 3 tablespoons olive oil
- 1 French baguette
- 250 grams Gruyère cheese
Instructions
- Pour two tablespoons of melted butter into the bottom of a slow cooker. Add 3 pounds of thinly sliced sweet onions, stirring to coat evenly in the butter.
- Cover and cook on high for 5 – 6 hours, stirring occasionally, until the onions are deeply caramelized and golden brown.
- Add beef broth, red wine, fresh thyme, and bay leaves. Stir well, cover, and cook on high for 30 – 45 minutes.
- In a small bowl, whisk 2 tablespoons of all-purpose flour with about ½ cup of hot broth from the crockpot until smooth. Stir the flour mixture back into the soup to slightly thicken.
- Add ½ teaspoon of balsamic vinegar for acidity. Taste and season with salt and freshly ground black pepper. The soup should be well-seasoned and slightly salty.
- Preheat the broiler. Slice the French baguette into ½-inch thick slices and place on a parchment-lined baking sheet. Brush both sides with olive oil. Broil on the top rack for 2 – 3 minutes per side, or until golden brown — watch closely to avoid burning.
- Remove the bay leaves and ladle hot soup into oven-safe bowls or crocks. Top each bowl with 2 – 3 toasted baguette slices, then cover generously with grated Gruyère cheese.
- Place bowls on the baking sheet and broil for 1 – 2 minutes or until the cheese is melted and bubbly. Carefully remove from the oven (the bowls will be hot!) and serve immediately with a few sprigs of fresh thyme on top for garnish.
Notes
Nutrition
Nutrition information is automatically calculated, so should only be used as an approximation.
