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thinly sliced onions caramelizing in a slow cooker, surrounded by fresh thyme, garlic, and a bottle of white wine, emphasizing the cooking process for French Onion Soup

Slow Cooker French Onion Soup

Kathleen ron
This Slow Cooker French Onion Soup is a classic French bistro-style dish made with slow-cooked caramelized onions, rich beef broth, and gooey melted Gruyère cheese over crispy baguette slices. Using a slow cooker enhances the depth of flavor while making the cooking process effortless. Perfect for cozy dinners, meal prepping, or entertaining guests, this comforting soup brings a taste of French cuisine to your home.
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Prep Time 15 minutes
Cook Time 7 hours
Total Time 7 hours 15 minutes
Course Soup
Cuisine French
Servings 5 people

Equipment

  • Slow Cooker For effortless cooking and deep flavor development.
  • Sharp Knife & Cutting Board For slicing onions evenly.
  • Wooden Spoon To stir the ingredients gently.
  • Oven-Safe Bowls Essential for broiling the cheese topping.
  • Baking Sheet To toast baguette slices.

Ingredients
  

For the Soup:

  • 6 large yellow onions thinly sliced
  • 3 tbsp butter unsalted
  • 6 cups beef broth or vegetable broth for vegetarian
  • ½ cup dry white wine e.g., Sauvignon Blanc
  • 2 cloves garlic minced
  • 1 sprig fresh thyme or ½ tsp dried thyme
  • 2 bay leaves
  • 1 tsp salt adjust to taste
  • ½ tsp black pepper

For the Topping:

  • 1 baguette sliced into 1-inch rounds
  • 2 cups Gruyère cheese shredded (or Swiss/Emmental as alternatives)
  • ¼ cup Parmesan cheese optional, for extra sharpness
  • 1 tbsp butter for brushing baguette slices

Optional Flavor Enhancers:

  • 1 tbsp brandy or cognac for depth
  • 1 tbsp balsamic vinegar for subtle sweetness
  • Extra thyme sprigs for garnish

Instructions
 

1️⃣ Prepare the Onions:

  • Peel and thinly slice 6 large yellow onions into even rounds.
  • In a large skillet, melt 3 tbsp butter over medium heat.
  • Add the onions and cook, stirring occasionally, for 10-15 minutes until soft and lightly golden. (This step enhances caramelization but can be skipped for a true hands-free slow cooker method.)
  • Add garlic and cook for another 1-2 minutes until fragrant.

2️⃣ Assemble the Ingredients in the Slow Cooker:

  • Transfer the caramelized onions to the slow cooker.
  • Pour in 6 cups of beef broth and ½ cup of white wine.
  • Add thyme, bay leaves, salt, and pepper.
  • Cover and cook on LOW for 6-8 hours (or HIGH for 4-5 hours).

3️⃣ Prepare the Cheese Toast Topping:

  • Preheat your oven to 375°F (190°C).
  • Arrange baguette slices on a baking sheet and brush lightly with butter.
  • Toast for 8-10 minutes until golden and crisp.

4️⃣ Assemble & Broil the Soup:

  • Remove thyme sprigs and bay leaves from the soup.
  • Ladle hot soup into oven-safe bowls placed on a baking sheet.
  • Top each bowl with a toasted baguette slice and a generous amount of shredded Gruyère cheese.
  • Broil in the oven for 2-3 minutes or until the cheese is melted, bubbly, and slightly browned.

5️⃣ Serve & Enjoy:

  • Garnish with fresh thyme sprigs if desired.
  • Serve immediately and enjoy the rich, comforting flavors!
  • Nutrition Facts (Per Serving)
  • 📊 Approximate Values
  • Calories: 380 kcal
  • Protein: 15g
  • Carbohydrates: 35g
  • Healthy Fats: 18g
  • Fiber: 4g
  • (Values may vary depending on ingredient substitutions and portion sizes.)

Recipe Notes & Pro Tips

  • 💡 Expert Tips for the Best French Onion Soup:
  • ✔ Caramelize the onions properly – This step enhances the soup’s depth of flavor.
  • ✔ Use high-quality broth – A rich beef stock makes a huge difference.
  • ✔ Choose the right cheese – Gruyère is the best choice for its nutty, melty texture.
  • ✔ Toast the bread well – It prevents sogginess when submerged in the soup.
  • ✔ Deglaze with wine or brandy – Adds complexity and enhances the flavors.

More Cozy Soup Recipes to Try

  • 🍲 Creamy Roasted Tomato Basil Soup – A comforting classic with rich flavors.
  • 🥣 Slow Cooker Butternut Squash Soup – A velvety fall favorite.
  • 🍜 Classic Chicken Noodle Soup – Perfect for cold days and easy meal prep.

Storage & Reheating Instructions

    How to Store Leftovers:

    • Refrigeration: Store in an airtight container for up to 4–5 days.
    • Freezing: Freeze the soup (without bread and cheese) in a freezer-safe container for up to 3 months.

    Best Reheating Methods:

    • On the Stovetop: Warm the soup over medium heat until hot.
    • In the Microwave: Heat in 1-minute intervals, stirring in between.
    • For the Topping: Re-toast baguette slices and broil cheese separately before assembling.

    Frequently Asked Questions

    • 1️⃣ Can I Make This Soup Without Wine?
    • Yes! Substitute with extra beef broth, white grape juice, or apple cider vinegar.
    • 2️⃣ What’s the Best Cheese for French Onion Soup?
    • Gruyère is the traditional choice, but Swiss, Emmental, or Fontina work well too.
    • 3️⃣ Can I Use Vegetable Broth Instead?
    • Yes! Swap beef broth for vegetable broth to make a vegetarian version.
    • 4️⃣ How Do I Get the Perfect Cheese Crust?
    • Broil until the cheese bubbles and turns golden brown for an authentic touch.
    • 5️⃣ What’s the Best Bread to Use?
    • A French baguette works best because it stays crisp even after soaking up the broth.
    Keyword Slow cooker French onion soup