Slow Cooker French Onion Soup: Easy & Classic Recipe

If you’d told my younger self I’d one day be making this Slow Cooker French onion soup at home, I never would’ve believed you. As a kid, I remember watching my grandma would always order a little crock of it at a cozy  French-themed café, and I didn’t get it. Other than the cheesy part. Brown broth and onions? No, thank you. Oh, how tastes evolve.

These days, I love French onion soup. And now I love making it, too. Especially with the help of my trusty slow cooker.

Over the years, I’ve tested multiple versions of this comforting classic and have adapted this recipe to make it my own from a few others including Julia Child’s version from Mastering the Art of French Cooking. Shoutouts to Tasty and Southern Living, too. 

This post solves a common home-cook problem: French onion soup can seem fussy or time-consuming, but it doesn’t have to be. With a slow cooker and a little planning, it becomes a realistic and rewarding option—even on a weeknight. I’ve done the testing, the tweaking, and the taste comparisons. All that’s left is for you to enjoy the warm, savory reward.

Jump to:

• Getting Started
• Ingredient Recommendations
• Ingredient Alternatives
• Finishing Touches
• Pairings
• Leftovers
• Recipe

Getting Started

The most time-consuming part? Chopping the onions. You’ll want about two pounds of yellow onions—this gives you roughly 8 cups once sliced. On a wooden cutting board, I top and tail the onion, slice it in half, and peel off the outer layers. Then, I use a chef knife to thinly slice the onions by hand. This seems easier and quicker to me than the clean-up of a slicing tool or gadget. But maybe I will try my mandoline sometime.

My bench scraper makes quick work of scooping up the slices when they’re ready.

After that, melt the butter and gather the rest of the ingredients. You can shred the cheese ahead of time (highly recommend this!) and store it in the fridge until the end. Slice the baguette once the onions are cooked and you’re getting close to assembly.

I use a 4-quart slow cooker. You can use as big as a 6-quart one.

Slow Cooker French Onion Soup Ingredient Recommendations

Onions

The recipe calls for yellow onions, and I agree. They’re consistent, affordable, and mellow beautifully when caramelized. You can use pre-bagged or weigh out large individual onions—just make sure you hit the two-pound mark. I’ve also tried Vidalia for a slightly sweeter version.

Wine

I use dry white wine (like Chardonnay or Sauvignon Blanc). Use what you have or what you enjoy drinking. You won’t use much, so the rest is perfect for pairing with your meal.

You can also use dry white vermouth. In a pinch, sherry vinegar adds brightness. 

Baguette

Store-bought or our homemade baguette recipe—your choice. A crusty baguette is the traditional topper. Just slice it when you’re almost ready to serve so it’s nice and fresh. 

Cheese

Gruyère is the classic choice. Swiss works (and is featured in Julia’s recipe) if that’s what’s available. In a pinch, Parmesan adds a sharp, salty finish. Sometimes my decision is made by what I can find already shredded at the grocery store. Since the Gruyère often comes in a block, I usually try to shred it ahead of time and then store it in the fridge until needed.

Slow Cooker French Onion Soup Ingredient Alternatives

Gluten-Free Option

Use a gluten-free bread or omit the bread entirely. You’ll still get all the oniony, cheesy goodness.

Slow Cooker French Onion Soup Finishing Touches

I serve this soup in small ceramic, broiler-safe ramekins. This size is perfect when the soup is a starter or part of a larger meal. I’ve used smaller ones too. Just work with and adjust based on what you have. (Someday I hope to have a larger set like these!)

Use a baking sheet under the ramekins when broiling to catch any cheese overflow and make clean-up easier.

Broiling Tip: I tend to get a little nervous when broiling. I set my oven rack somewhere between the middle and top, depending on the ramekin height and how I feel about broiling that day.  I always set a timer and watch it closely. It’s too easy to walk away and get distracted. For me anyway.

Slow Cooker French Onion Soup Pairings

I will be making this to celebrate Bastille Day in July, but it also makes an appearance in my kitchen every November for Beaujolais Nouveau Day. It’s great for cozy nights, dinner parties, or when you want to bring a little French flair to your table.

Serve alongside:

• A fresh green salad
• Store-bought French baguette or our freshly made baguette recipe
• Slow Cooker Boeuf Bourguignon if you’re going for a full-on French feast
• A glass of the white wine used in the soup (or a nice red if you’re pairing it with the beef dish)
• A French playlist on Spotify for ambiance (Édith Piaf or Carla Bruni always hits the spot)

Slow Cooker French Onion Soup Leftovers

Great news—leftovers reheat beautifully. I warm the soup on the stovetop, then ladle it into ramekins, top with bread and cheese, and broil. Voilà—just like fresh.

I haven’t tried freezing it yet, but when I do, I’ll update here with the results.

French onion soup doesn’t have to be a restaurant-only treat or a special occasion ordeal. With a slow cooker and a few smart shortcuts, it’s totally doable at home—even for two. I hope this gives you the confidence to give it a try.

Let me know in the comments if you have a favorite version—or if you’re planning to make this Slow Cooker French Onion Soup for Bastille Day too!

Bon appétit!