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But here’s the good news: your slow cooker can handle most of the heavy lifting. This version delivers deep, rich, caramelized onion flavor with minimal active effort, just a bit of patience and one game-changing overnight rest in the fridge.
The secret to extraordinary depth? Caramelize the onions the day before. Let them slowly transform into sweet, golden perfection in the slow cooker, then refrigerate overnight. That extra chill time works magic, melding flavors in a way that makes the final soup taste like it simmered for days. The next day, simply add the remaining ingredients and let the slow cooker finish the job.
Traditionally, French onion soup is served with a toasted slice of baguette floating on top beneath a blanket of melted Gruyère cheese. This recipe, however, is so wonderfully onion-rich that I’ve found you can skip the bread entirely if you’d like. Simply top the soup with the cheese mixture, broil until golden and bubbly, and you’ll still enjoy every bit of that classic French onion soup experience while saving a few calories.
Whether you serve it as a cozy weeknight dinner or as the first course for a special gathering, this soup is well worth the wait.
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Slow Cooker French Onion Soup
For the Soup
4 T. butter
3 lbs. Melissa’s yellow onions, thinly sliced
1 Melissa’s red onion, thinly sliced
1 T. sugar
2 cloves garlic, minced
1 T. Worcestershire sauce
1 T. smoked paprika
½ t. freshly ground black pepper
Salt, to taste
½ c. dry sherry
½ c. dry red wine
8 c. beef broth
1 t. sea salt, or to taste
¼ t. dried thyme
1 bay leaf
½ c. flour
Water, enough to make a smooth slurry
½ c. heavy cream
For the Topping
8 slices French baguette
1 c. shredded Gruyère cheese
⅓ c. shredded Emmental cheese
¼ c. freshly grated Parmesan cheese
¼ c. shredded mozzarella cheese
Caramelize the Onions
Set a 6-quart slow cooker to Low. Place the butter in the bottom of the slow cooker and allow it to melt while you slice the onions. (A mandoline makes quick work of this.)
Add the onions and garlic to the melted butter. Sprinkle with the Worcestershire sauce, smoked paprika, steak seasoning, pepper, and sugar. Stir once to combine.
Cover and cook on Low for 10 to 12 hours. Stir occasionally if you happen to be nearby, but it's not essential.
During the final two hours, remove the lid to allow excess moisture to evaporate and help the onions deepen in color.
At this point, the caramelized onions can be refrigerated overnight.
Finish the Soup
Pour the sherry, red wine, and beef broth over the onions. Stir in the sea salt, thyme, and bay leaf. Cover and cook on Low for 8 to 10 hours or High for 4 to 6 hours.
During the last hour of cooking, whisk the flour with enough water to make a smooth slurry. Stir it into the soup to lightly thicken the broth.
When the soup is finished cooking, stir in the heavy cream and heat through.
Assemble
Preheat the broiler and position an oven rack about 8 inches from the heat source.
Toast the baguette slices under the broiler until lightly browned on both sides, about 1 to 2 minutes per side.
Combine the Gruyère, Emmental, Parmesan, and mozzarella cheeses.
Ladle the soup into oven-safe crocks, filling each about three-quarters full. Float a toasted baguette slice on top, if desired, and generously cover with the cheese mixture.
Broil until the cheese is melted, bubbling, and lightly browned, about 2 minutes.
Serve immediately.
NOTES
If you’re serving this soup to company, consider creating a “French onion soup bar.” Set out bowls of hot soup, toasted baguette slices, and the cheese mixture separately so everyone can customize their own bowl. Some guests will want lots of bread, others extra cheese, and a few may skip the bread altogether. Giving everyone the choice makes serving easy and adds a fun, interactive touch to the meal.
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