Caramelized Onion Soup is a warm, hearty bowl filled with sweet, slowly cooked onions that melt into a rich, flavorful broth. This soup brings out the deep, natural sweetness of the onions, paired perfectly with a touch of herbs and a cozy, comforting feel. It’s the kind of dish that feels special but is actually pretty simple to make.
I love taking my time caramelizing the onions just right—this step really brings the soup to life. Whenever I make it, the smell that fills the kitchen is so amazing it usually draws everyone in for a taste before it’s even done. A little patience here turns ordinary onions into something truly delicious and worth every minute.
My favorite way to serve this soup is with a crusty slice of bread topped with melted cheese, making it perfect for dipping and scooping. It’s an easy meal to come home to on a chilly day, and it always feels like a warm hug in a bowl. I’m pretty sure this soup has won over even the pickiest eaters in my family because of that lovely balance between sweet and savory.
Key Ingredients & Substitutions
Yellow Onions: These onions are perfect for caramelizing because they become sweet and rich after slow cooking. If you don’t have yellow onions, sweet onions or Vidalia work well too.
Butter & Olive Oil: Using both adds depth of flavor and helps with browning. You can replace butter with all olive oil if you prefer dairy-free.
Beef Broth: It adds a rich, deep flavor to the soup. For a vegetarian option, vegetable broth works great but maybe use a bit of soy sauce or miso for extra umami.
Gruyère Cheese: This cheese melts beautifully and adds nuttiness. Swiss cheese or mozzarella are good alternatives, though milder in flavor.
Baguette: A crusty bread is best for soaking up the soup. Try sourdough or any sturdy bread you have on hand.
How Can You Perfectly Caramelize Onions for Soup?
Caramelizing onions is all about patience. Cooking them low and slow lets their natural sugars develop deep flavors without burning.
- Use a wide, heavy-bottom pan to cook evenly and prevent sticking.
- Start on medium heat, then lower to medium-low or low after the onions start to soften.
- Stir often but not constantly—every few minutes to prevent burning.
- Add a pinch of sugar early on to speed up caramelization.
- It can take 40-50 minutes, but the sweet, golden color is well worth the wait!
Equipment You’ll Need
- Heavy-bottomed large pot – this helps the onions cook evenly without burning.
- Wooden spoon – gentle on your pot and great for stirring caramelizing onions slowly.
- Sharp chef’s knife – makes slicing onions thin and uniform quick and safe.
- Oven-safe soup bowls or crocks – needed to broil the cheese-topped soup safely.
- Baking sheet – to toast the bread and hold soup bowls under the broiler.
Flavor Variations & Add-Ins
- Use sherry or brandy instead of white wine for a richer, deeper flavor.
- Add sliced cremini or button mushrooms while caramelizing for a meaty twist.
- Top with different cheeses like cheddar or fontina for different melt and flavor profiles.
- Sprinkle with fresh herbs like rosemary or sage in place of thyme for a new herbal note.
How to Make Caramelized Onion Soup
Ingredients You’ll Need:
- 4 large yellow onions, thinly sliced
- 3 tablespoons unsalted butter
- 1 tablespoon olive oil
- 1 teaspoon salt
- ½ teaspoon sugar
- 4 cloves garlic, minced
- 2 tablespoons all-purpose flour
- 8 cups beef broth (or vegetable broth for vegetarian)
- ½ cup dry white wine (optional)
- 2 teaspoons fresh thyme leaves (plus extra for garnish)
- Salt and freshly ground black pepper to taste
- 1 baguette or crusty bread, sliced
- 2 cups grated Gruyère or Swiss cheese
How Much Time Will You Need?
This recipe takes about 1 hour and 15 minutes total. Most of the time is spent slowly caramelizing the onions for 40-50 minutes, then simmering the soup for 25-30 minutes. Toasting the bread and broiling the cheese-topped soup add about 10 minutes at the end.
Step-by-Step Instructions:
1. Caramelize the Onions:
Heat the butter and olive oil in a large heavy-bottomed pot over medium heat. Add the sliced onions and salt. Cook slowly, stirring occasionally, until the onions turn deep golden and caramelized—this takes about 40 to 50 minutes. After about 10 minutes, sprinkle the sugar over the onions to help with caramelization.
2. Build the Soup Base:
Once the onions are caramelized, add the minced garlic and cook for another minute until fragrant. Sprinkle the flour over the onions and stir continuously for 2-3 minutes to cook out the raw flour taste. Pour in the white wine (if using) and scrape the pot bottom to lift those delicious browned bits.
3. Simmer the Soup:
Add the broth and fresh thyme to the pot. Stir well and bring the soup to a simmer. Let it cook gently for 25 to 30 minutes so the flavors meld together. Taste and season with salt and pepper as needed.
4. Prepare the Topping and Serve:
While the soup simmers, preheat your oven to 400°F (200°C). Arrange the bread slices on a baking sheet and toast until crisp and golden, about 5 to 7 minutes. Ladle the hot soup into oven-safe bowls, place a toasted bread slice on top of each, and cover with a generous amount of grated Gruyère cheese. Place the bowls on a baking sheet and broil in the oven for 3 to 5 minutes until the cheese bubbles and browns. Garnish with fresh thyme leaves and serve right away.
Can I Use Frozen Onions for This Soup?
Fresh onions work best for caramelizing, but if you have frozen onions, make sure to thaw and drain them well before cooking. Keep in mind frozen onions may release more moisture, so cook a bit longer to achieve that deep caramelized flavor.
Can I Make Caramelized Onion Soup Ahead of Time?
Absolutely! You can caramelize the onions and make the soup up to 2 days in advance. Store it in an airtight container in the fridge and reheat gently on the stove before adding the bread and cheese topping.
How Should I Store Leftovers?
Keep any leftover soup in a sealed container in the fridge for up to 3 days. When reheating, warm it on the stove and add the bread and cheese fresh before serving to keep the toppings crisp and melty.
What Can I Substitute for Gruyère Cheese?
If you don’t have Gruyère, Swiss cheese or Fontina make great alternatives. They melt well and offer a mild, nutty taste that complements the soup perfectly.
