French Onion Soup

French Onion Soup is like caramelized onions transformed into a comforting soup, crowned with cheesy bread. The French really know their flavors! Caramelizing onions takes some patience, but it’s well worth the effort. The result? Deeply golden, sweet onions that create the rich, flavorful foundation of this iconic soup. If you want a hands-off approach, you can even caramelize the onions in a slow cooker.

Despite its grand reputation, French Onion Soup is made with just a handful of ingredients: butter, onions, flour, and stock or broth. The magic happens when the onions are slowly caramelized for at least 40 minutes. And of course, the star on top is the cheesy toast! Because let’s be honest, without that melted, glorious cheese toast, it’s just onion soup. And that’s not nearly as exciting!

The caramelized onions are the heart and soul of this soup, right alongside the cheesy toast. While caramelizing onions on the stovetop takes around 45 minutes, it’s not a difficult process. Most of the time, they cook over low heat, and you just stir them occasionally. It’s the perfect recipe to work on while you’re busy with other kitchen tasks. My cast iron pot, which is 24 cm (10 inches) wide, was overflowing with raw onions and took almost an hour to caramelize. If you have a wider pot, you might finish closer to 45 minutes.

Why You’ll Love This French Onion Soup

Ingredients for French Onion Soup

1. Unsalted Butter (100 g / 3.5 oz):
   Using unsalted butter allows you to control the saltiness of the dish. It also adds a rich, creamy flavor to the base of the soup when cooking the onions.
2. Onions (1.25 kg / 2.5 lb):
   Both brown and yellow onions are ideal for caramelizing, as they offer a sweet and robust flavor. Make sure to slice them thinly from top to bottom for even cooking and a great texture.
3. Dry White Wine (¾ cup / 185 ml, optional):
   Adds depth and acidity to the soup, balancing the richness of the onions and butter. If you prefer not to use wine, you can omit it or replace it with additional broth.
4. All-Purpose Flour (2 tbsp):
   Helps thicken the soup slightly, giving it a more velvety texture. Be sure to cook the flour with the onions for a minute or two to avoid a raw flour taste.
5. Beef or Chicken Broth (1.5 litres / 6 cups):
   The choice between beef or chicken broth will affect the flavor. Beef broth brings a deeper, more robust taste, while chicken broth offers a lighter, milder flavor. Use a low-sodium version to prevent over-salting.
6. Bay Leaves (2 dried or 3 fresh, optional):
   Adds a subtle, earthy flavor. Dried bay leaves are stronger in taste, so use fewer if using fresh ones.
7. Fresh Thyme (3 sprigs, optional):
   Fresh thyme contributes a hint of herbal flavor. It pairs wonderfully with the sweetness of caramelized onions, but can be omitted if not available.
8. Salt and Pepper:
   Adjust the seasoning at the end to taste, especially considering the salt level of your broth.

Cheesy Toast:

1. French Baguette (6-8 slices):
   A classic choice for French onion soup. The baguette should be sliced thick enough to hold up when placed in the soup but still soft enough to soak up the liquid.
2. Gruyere or Mozzarella Cheese (100 g / 3.5 oz):
   Gruyere is the traditional choice for its nutty, salty flavor and excellent melting quality. Mozzarella can be used as a more mild, stretchy alternative. Feel free to use any good melting cheese you prefer.
3. Fresh Parsley or Thyme (optional garnish):
   Adds a touch of color and freshness to the finished dish, balancing the richness of the soup.

How To Make This French Onion Soup

Caramelize the Onions:

1. Melt the butter in a heavy-bottomed casserole pot over medium heat. Once it begins to foam, add the onions and stir to coat them evenly in the butter.
2. Cook on medium-low: Reduce the heat to medium-low and cook for 30 minutes, stirring occasionally, until the onions are softened and translucent.
3. Add salt and cook on medium-high: Increase the heat to medium or medium-high. Sprinkle with salt and cook for another 20-30 minutes, stirring more frequently, until the onions turn a deep golden brown and develop a sweet flavor.

Make the Soup:

1. Deglaze: Pour in the wine and simmer briskly for 2 minutes, allowing most of the liquid to evaporate while scraping up the browned bits from the bottom of the pot.
2. Add flour: Sprinkle the flour over the onions and cook for 1 minute.
3. Add broth and herbs: Pour in the broth and add thyme and bay leaves.
4. Simmer: Cover the pot and lower the heat, letting it simmer gently for 30 minutes.
5. Serve: Season the soup with salt and pepper to taste. Ladle into bowls and top each with 1 or 2 slices of cheesy toast.

Cheese Toast:

1. Preheat the grill/broiler to high. Toast both sides of the bread until lightly golden.
2. Top the bread with cheese and grill until the cheese melts and develops golden brown spots.

Storage Options

Variations and Substitutions

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French Onion Soup

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Prep Time: 15 minutes minutes

Cook Time: 1 hour hour 30 minutes minutes

Servings: 4

Ingredients

• 100 g 3.5 oz unsalted butter
• 1.25 kg 2.5 lb brown or yellow onions, peeled, halved, and thinly sliced lengthwise (see Note 1)
• 3/4 cup 185 ml dry white wine (optional, see Note 2)
• 2 tbsp plain/all-purpose flour
• 1.5 litres 6 cups or 1.5 quarts good-quality low-sodium beef or chicken broth (see Note 3)
• 2 dried bay leaves or 3 fresh ones, optional
• 3 sprigs fresh thyme optional
• Salt and pepper to taste
• For the Cheesy Toast
• 6 to 8 slices of French baguette or similar soft bread see Note 4
• 100 g 3.5 oz Gruyere, mozzarella, or another melting cheese of your choice
• Fresh parsley or thyme leaves for garnish (optional)

Instructions

• Prepare the Onions:
• In a heavy-based casserole pot, melt the butter over medium heat. When it begins to foam, add the onions and stir to coat them in butter.
• Reduce the heat to medium-low and cook for about 30 minutes, stirring occasionally, until the onions soften and turn semi-transparent.
• Increase the heat to medium-high, add a pinch of salt, and cook for an additional 20–30 minutes, stirring more frequently, until the onions are deep golden and caramelized.
• Make the Soup:
• Deglaze the pot by adding the wine and simmering for 2 minutes, scraping up any bits stuck to the bottom until most of the liquid has evaporated.
• Sprinkle the flour over the onions and cook for 1 minute.
• Add the broth, thyme, and bay leaves, stirring well.
• Cover the pot, lower the heat to a gentle simmer, and cook for 30 minutes.
• Taste and season with salt and pepper. Ladle the soup into bowls and top with cheesy toast slices.
• Prepare the Cheesy Toast:
• Preheat your grill or broiler to high. Lightly toast both sides of the bread slices until golden.
• Top the toasted bread with cheese and grill until melted and bubbly with brown spots.
• Serve: Garnish with fresh parsley or thyme if desired, and serve with cheesy toast on top of the soup.