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Slow Cooker French Onion Soup Recipe

Slow Cooker French Onion Soup

Wholesome Cove
This slow cooker French onion soup takes the fuss out of the classic bistro dish. The slow cooker does the patient work, transforming onions and broth into a deeply savory, rich soup that tastes like it simmered all day.
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Prep Time 25 minutes
Cook Time 8 hours 35 minutes
Total Time 9 hours
Course Soup
Cuisine French
Servings 6 bowls
Calories 385 kcal

Equipment

  • Slow cooker (5-quart or larger) - The heart of this recipe
  • Large skillet or enameled Dutch oven - For caramelizing the onions
  • Mandoline slicer - Optional, for fast, even onion slices
  • Cutting board
  • Chef's knife
  • Serrated bread knife - For clean baguette rounds
  • Box grater - For shredding cheese
  • Oven-safe French onion soup bowls - Or any broiler-safe crocks
  • Rimmed baking sheet - To hold bowls under the broiler
  • Ladle - For serving
  • Wooden spoon - For stirring the onions

Ingredients
  

  • 3 lbs yellow onions - 1.4 kg, about 5 to 6 large, thinly sliced
  • 4 tbsp unsalted butter - 57g
  • 2 tbsp olive oil - 30ml
  • 1 tsp granulated sugar - optional, helps caramelize
  • 1 tsp salt - plus more to taste
  • 1/2 tsp freshly ground black pepper
  • 4 cloves garlic - minced
  • 3 tbsp all-purpose flour - 24g
  • 8 cups beef broth - 1.9 L, or beef stock
  • 1/2 cup dry white wine - 120 ml, or dry sherry, optional
  • 2 tbsp Worcestershire sauce - 30 ml
  • 1 tbsp balsamic vinegar - 15 ml
  • 1 tbsp fresh thyme leaves - or 1 tsp dried thyme
  • 2 bay leaves

For the Topping

  • 1 French baguette - sliced into 1/2-inch rounds
  • 2 cups Gruyère cheese - 200g, freshly shredded
  • 1/2 cup Swiss or provolone cheese - 50g, shredded, optional for extra pull

Instructions
 

  • Slice the onions: Peel and trim onions, then slice into thin half-moons about 1/8 inch thick. Mince the garlic and set aside.
  • Caramelize the onions: Melt butter with oil in a large skillet over medium heat. Add onions, sugar, salt, and pepper. Cook for 25-30 minutes, stirring occasionally, until deep golden brown and jammy.
  • Add garlic and flour: Stir in minced garlic and cook for 1 minute until fragrant. Sprinkle in flour and stir constantly for 2 minutes to cook out the raw taste.
  • Transfer to slow cooker: Scrape the onion mixture into the slow cooker. Add beef broth, wine (if using), Worcestershire sauce, balsamic vinegar, thyme, and bay leaves. Stir to combine.
  • Slow cook the soup: Cover and cook on LOW for 8 hours or HIGH for about 4 hours, until deeply flavored.
  • Season and finish: Remove bay leaves. Taste and adjust salt, pepper, Worcestershire, or balsamic as needed.
  • Toast the baguette: Slice baguette into rounds. Toast on a baking sheet at 350°F (175°C) for 6-8 minutes until dry and lightly crisp.
  • Assemble and broil: Ladle hot soup into oven-safe bowls. Float toasted baguette slices on top. Pile on shredded Gruyère and optional Swiss/provolone. Broil for 2-4 minutes until bubbling and browned.
  • Serve: Let bowls rest for a couple of minutes, then serve immediately with extra bread on the side.

Notes

  • Choose the right onions: Yellow onions offer the best balance of sweetness and bite. Sweet onions like Vidalia can make the soup too sugary.
  • Caramelize patiently: Don't stop at soft and pale; push the onions to a deep, rich brown for maximum flavor. This is the most important step.
  • Shred your own cheese: Pre-shredded cheese has anti-caking agents that prevent smooth melting. Freshly grated Gruyère gives the best, stretchiest results.
  • Toast the bread well: A dry, crisp baguette round holds up in the broth. Day-old bread is even better as it won't turn to mush as quickly.
  • Season at the end: Broth sodium levels vary widely. Wait until the soup has finished cooking to adjust the salt and pepper to your taste.
  • Make-ahead and storage: The soup base (without bread and cheese) can be refrigerated for up to 4 days or frozen for 3 months. Finish with fresh bread and cheese when serving.
  • Fully hands-off variation: For an even lazier version, combine sliced onions, butter, and salt in the slow cooker and cook on LOW for 10-12 hours (overnight) until caramelized, then proceed with the broth.
  • Vegetarian option: Swap beef broth for a rich mushroom or vegetable stock and use a vegetarian Worcestershire sauce. The result is still deeply savory.
  • Boozy depth: For a more robust flavor, replace the white wine with a splash of brandy or cognac.
  • Don't skip deglazing: Scraping the browned bits (fond) from the skillet into the slow cooker adds immense flavor to the broth.

Nutrition

Serving: 1BowlCalories: 385kcalCarbohydrates: 34gProtein: 17gFat: 19gSaturated Fat: 10gPolyunsaturated Fat: 1gMonounsaturated Fat: 7gCholesterol: 48mgSodium: 980mgPotassium: 540mgFiber: 4gSugar: 11gVitamin A: 6IUVitamin C: 15mgCalcium: 320mgIron: 10mg
Keyword caramelized onion soup, comfort food soup, easy french onion soup, slow cooker french onion soup
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