Caramelized Onion Soup is a warm and comforting bowl of sweet, slow-cooked onions simmered in a rich broth that’s both simple and deeply satisfying. The heart of the soup is those tender, golden-brown onions that bring a natural sweetness and mellow flavor, making every spoonful feel like a cozy hug on a chilly day.
I love how patient you have to be while cooking the onions low and slow until they develop that beautiful caramel color—that’s when all the magic happens. It’s worth every minute because the flavor deepens and transforms completely. I usually make a big batch, so I have enough leftovers to enjoy the next day or to freeze for a quick nibble anytime.
My favorite way to serve this soup is with a slice of toasted bread topped with melted cheese, right out of the oven—there’s just something about the crispy, gooey bread that makes each bite extra special. It’s a classic combo, and everyone in my house looks forward to that bubbly cheese crust on top. This soup always brings people together around the table, sharing warmth and simple comfort.
Key Ingredients & Substitutions
Yellow Onions: These are best for caramelizing thanks to their natural sweetness that deepens when cooked slowly. If you can’t find yellow onions, sweet onions or even shallots can work but adjust cooking time as they caramelize faster.
Butter & Olive Oil: Butter adds rich flavor while olive oil helps prevent burning. You can use all butter or all oil if you prefer. For dairy-free, swap butter with a plant-based option.
Broth: Beef broth gives deep, hearty flavor, but vegetable broth works well for a vegetarian version. Make sure to use low-sodium broth to control saltiness.
Gruyère Cheese: This cheese melts beautifully and adds a nutty taste. Swiss cheese is a great substitute. For a different twist, mozzarella or fontina also melt nicely.
Baguette: A sturdy baguette holds up well when toasted and topped with cheese. You can use sourdough or a crusty Italian bread as an alternative.
How Do You Get Perfectly Caramelized Onions Without Burning?
Caramelizing onions is the heart of this soup and needs patience and attention. Here’s how to nail it:
- Cook onions slowly over medium-low heat with butter and oil to prevent burning.
- Stir often, scraping the pan bottom to release sticky bits that add flavor.
- Add a pinch of salt and a little sugar early on to boost sweetness and help draw out moisture.
- Adjust heat if onions start sticking or browning too fast—lower the heat and keep stirring.
- The process usually takes 40-50 minutes; don’t rush it! The onions should be soft, very dark golden, and jammy.
- Once caramelized, adding garlic and deglazing with wine lifts the flavor even more.
Equipment You’ll Need
- Large heavy-bottomed pot or Dutch oven – it spreads heat evenly for perfect caramelization without burning.
- Wooden spoon or heatproof spatula – great for stirring the onions gently so they cook evenly.
- Chef’s knife – sharp and sturdy for slicing onions thin and uniform.
- Oven-safe bowls or ramekins – needed if you want to broil the cheese-topped soup for a bubbly finish.
- Baking sheet – for toasting the baguette slices before topping with cheese.
- Cheese grater – to shred your Gruyère or Swiss cheese finely for even melting.
Flavor Variations & Add-Ins
- Use yellow or red onions together for a mix of sweetness and a slightly sharper bite.
- Add a splash of sherry or brandy instead of white wine for a richer, deeper flavor.
- Stir in fresh herbs like rosemary or bay leaves while simmering for extra aroma.
- Top the soup with different cheeses like fontina or mozzarella for a milder, creamier cheese crust.
How to Make Caramelized Onion Soup?
Ingredients You’ll Need:
For the Soup:
- 4 large yellow onions, thinly sliced
- 3 tablespoons unsalted butter
- 1 tablespoon olive oil
- 1 teaspoon salt
- ½ teaspoon sugar
- 4 cups beef broth (or vegetable broth for vegetarian)
- ½ cup dry white wine (optional)
- 2 cloves garlic, minced
- 1 teaspoon fresh thyme leaves (plus extra for garnish)
- 1 bay leaf
- Freshly ground black pepper, to taste
For the Topping:
- 1 baguette, sliced into ½ inch slices
- 1 ½ cups grated Gruyère cheese (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 (about 40-50 minutes), plus simmering the soup for 20-30 minutes. Toasting the bread and melting the cheese under the broiler will take an additional 10 minutes.
Step-by-Step Instructions:
1. Caramelizing the Onions:
In a large pot or Dutch oven, melt the butter with the olive oil over medium heat. Add the sliced onions, salt, and sugar. Cook the onions slowly, stirring frequently, until they turn deep golden brown and caramelize. This takes about 40-50 minutes. Keep an eye and stir often to prevent burning.
2. Building the Soup Base:
After the onions have caramelized, add the minced garlic and cook for 1-2 minutes until fragrant. Pour in the white wine if using, scraping any browned bits off the bottom of the pot. Let it simmer for about 5 minutes. Then add the beef broth, thyme, bay leaf, and pepper to taste. Bring everything to a simmer and let cook for 20-30 minutes. Remove bay leaf before serving.
3. Preparing and Serving:
While the soup simmers, preheat the oven to 375°F (190°C). Toast the sliced baguette on a baking sheet in the oven until lightly crisp, about 5-7 minutes. Remove bread, top with a generous amount of grated Gruyère cheese, and broil just until the cheese is melted, bubbly, and golden. Ladle hot soup into oven-safe bowls, place the cheesy toast on top, and garnish with fresh thyme. For an extra cheesy crust, place the bowls under the broiler for 2-3 minutes before serving.
Can I Use Frozen Onions for This Soup?
It’s best to use fresh onions for caramelizing since frozen ones release extra moisture and don’t caramelize as well. If you must use frozen, thaw completely and pat dry before cooking, but expect a different texture and flavor.
How Do I Store Leftover Caramelized Onion Soup?
Keep leftovers in an airtight container in the fridge for up to 3 days. Reheat gently on the stove over low heat to avoid burning the onions. You can also freeze the soup for up to 2 months; thaw overnight in the fridge before reheating.
Can I Skip the Wine in This Recipe?
Yes! The wine adds depth but is optional. If you leave it out, simply deglaze the pot with a bit of broth or water to lift the flavorful browned bits from the pan.
What’s the Best Bread for Topping the Soup?
A sturdy crusty baguette works best since it holds up well under the cheese and in the soup. You can also use sourdough or Italian bread slices; just make sure to toast them before adding cheese to get that perfect crispy base.
