French Soupe à l'Oignon Gratinée

Name: French Soupe à l'Oignon Gratinée
Prep time: 15 min
Cook time: 55 min
Total time: 1 h 10 min
Yield: 4 servings
Cuisine: French
Ingredients: 3 lbs (1360 g) yellow onions, peeled and sliced into thin half-moons (about ¼-inch thick), peeled and sliced into thin half-moons; 3 tablespoons (43 g) unsalted butter, cold; 1 tablespoon (15 ml) extra-virgin olive oil; to taste kosher salt; to taste freshly ground black pepper; 6 cups (1440 ml) vegetable broth, preferably homemade or high-quality, warm; ½ cup (120 ml) dry white wine (such as Sauvignon Blanc or Pinot Grigio); 2 sprigs fresh thyme sprigs; 1 leaf bay leaf; 8 slices (150 g) baguette or rustic white bread, sliced into ½-inch-thick rounds, sliced into ½-inch-thick rounds, lightly toasted; 2 cups (200 g) Gruyère cheese, finely grated or shredded, finely grated

From the MyKitchenBank Kitchen

A classic French vegetarian onion soup with a rich, deeply caramelized onion base, finished under the broiler with crusty bread and melted Gruyère cheese. The long, slow cooking transforms simple onions into a luxurious golden broth—a rustic bistro favorite that's both humble and elegant.

Prep: 15 min
Cook: 55 min
Wait: 45 min
Total: 1 hr 55 min
Makes: 4 servings (about 1.5 cups per bowl)

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Ingredients

4 servings

3 lbs (1360 g) yellow onions, peeled and sliced into thin half-moons (about ¼-inch thick), peeled and sliced into thin half-moons
3 tablespoons (43 g) unsalted butter, cold
1 tablespoon (15 ml) extra-virgin olive oil
to taste kosher salt
to taste freshly ground black pepper
6 cups (1440 ml) vegetable broth, preferably homemade or high-quality, warm
½ cup (120 ml) dry white wine (such as Sauvignon Blanc or Pinot Grigio)
2 sprigs fresh thyme sprigs
1 leaf bay leaf
8 slices (150 g) baguette or rustic white bread, sliced into ½-inch-thick rounds, sliced into ½-inch-thick rounds, lightly toasted
2 cups (200 g) Gruyère cheese, finely grated or shredded, finely grated

Equipment (7)

Large heavy-bottomed pot or Dutch oven (3-quart minimum)
Sharp chef's knife
Cutting board
Wooden spoon for stirring
4 oven-safe soup bowls or crocks (10–12 oz each)
Shallow baking sheet
Cheese grater

Steps

Before you start

Use a heavy-bottomed pot or Dutch oven (at least 3 quarts) so onions cook evenly without burning
Have your vegetable broth ready and warm (not cold) to add to the caramelized onions
Prepare oven-safe bowls or ramekins that can handle broiler heat; you'll use these for serving
Slice onions uniformly so they cook at the same rate

1
Prepare the onions
- Slice the 3 lbs (1360 g) of yellow onions into thin, even half-moons about ¼ inch thick.
- This uniform size ensures they caramelize evenly.
- Set aside.
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Tip: Use a sharp chef's knife or mandoline for consistent slices. Thinner onions cook faster and caramelize better.
2
Start caramelizing onions
- Heat a large heavy-bottomed pot (3-quart or larger) over medium heat.
- Add 3 tablespoons (43 g) of unsalted butter and 1 tablespoon (15 ml) of olive oil.
- Once the butter foams and the oil shimmers, add all the sliced onions.
- Stir well to coat them evenly with the fat.
- Cook for about 5 minutes, stirring occasionally, until they begin to soften and release their moisture.
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Ready when: Onions are softened and release clear liquid
Tip: Use both butter and oil—butter adds flavor while oil raises the smoke point so nothing burns.
3
Continue caramelizing
- Lower the heat to medium-low.
- Continue cooking the onions, stirring every 5–7 minutes with a wooden spoon.
- The onions will gradually turn golden, then amber, then deep brown.
- This process takes 30–40 minutes.
- Scrape up any browned bits stuck to the bottom of the pot (fond) when you stir, as this adds rich flavor.
- The onions are ready when they're a deep mahogany color and very soft, almost falling apart.
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Ready when: Onions are deep mahogany brown, very soft, and heavily reduced
Tip: Don't skip the stirring—low-and-slow is key. Stir every 5–7 minutes to prevent burning and encourage even caramelization.
4
Deglaze with wine
- Increase the heat to medium.
- Pour in ½ cup (120 ml) of dry white wine, scraping the bottom of the pot with your wooden spoon to dissolve all the caramelized fond.
- Let the wine bubble gently for 2–3 minutes, stirring occasionally, until reduced by about half and the sharp alcohol smell is gone.
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Ready when: Wine is reduced by half, alcohol aroma is gone
Tip: Deglazing captures all those flavorful browned bits—don't skip this step.
5
Add broth and aromatics
- Add 6 cups (1440 ml) of warm vegetable broth, along with 2 thyme sprigs and 1 bay leaf.
- Stir well.
- Bring the mixture to a gentle simmer over medium heat, then reduce the heat to low.
- Simmer gently, partially covered, for 15 minutes.
- This allows the flavors to meld.
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Ready when: Soup is at a gentle, steady simmer
Tip: Warm broth integrates faster and more smoothly into the caramelized onions than cold broth.
6
Season the soup
- Remove the thyme sprigs and bay leaf.
- Taste the soup and season with kosher salt and freshly ground black pepper to your preference.
- Remember that the cheese will add saltiness, so season a bit conservatively.
- The soup should be rich, deeply flavored, and taste primarily of caramelized onions.
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Tip: Taste as you go—you can always add more salt, but you can't take it out.
7
Toast the breadOven 500°F / 260°C
- While the soup finishes, preheat your broiler on high.
- Arrange 8 baguette slices (each about ½ inch thick) on a shallow baking sheet.
- Toast under the broiler for 1–2 minutes, until golden and crispy on top.
- Watch carefully so they don't burn.
- Remove and set aside.
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Ready when: Bread is golden and crispy on top
Tip: Don't walk away—bread toasts very quickly under the broiler and can scorch in seconds.
8
Assemble the gratinée
- Arrange 4 oven-safe soup bowls or crocks on a shallow baking sheet.
- Ladle the hot soup into each bowl, filling it about ¾ full (roughly 1½ cups per bowl).
- Place 2 toasted baguette slices on top of each bowl of soup, overlapping slightly.
- Divide the 2 cups (200 g) of grated Gruyère cheese evenly among the bowls, piling it generously on top of the bread.
- Press down gently so the cheese adheres.
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Tip: Use oven-safe bowls—ceramic crocks or proper soup crocks work best. Make sure the handles (if any) can take broiler heat.
9
Broil until goldenOven 500°F / 260°C
- Place the baking sheet with the bowls under the preheated broiler, positioned 4–6 inches from the heat source.
- Broil for 3–5 minutes, until the cheese is melted, bubbly, and golden brown on top.
- Some charring on the cheese edges is desirable—it adds flavor.
- Watch carefully; cheese can go from perfect to burnt very quickly under the broiler.
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Ready when: Cheese is melted, bubbly, and golden to light brown on top
Tip: Position bowls 4–6 inches from the broiler element. Too close and the cheese burns; too far and it won't brown.
10
Serve immediately
- Carefully remove the baking sheet from the broiler (the bowls will be very hot).
- Transfer each bowl to a serving plate or place it on a small trivet.
- Let rest for 1 minute before serving so the inside cools slightly and is safe to eat.
- Serve with small spoons and warn guests that the bowls are extremely hot.
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Tip: The bowls stay hot for several minutes. Handle with care and always warn guests.

Good to know

Tips

Patience is everything: true soupe à l'oignon gratinée depends on slow, deep caramelization of the onions (30–40 minutes). Rushing this step or cooking on too-high heat will give you light onion soup, not the rich, complex dish this recipe aims for.
Caramelize in a wide, heavy pot so onions spread in a single layer and heat evenly. A thin pot or small surface area leads to steaming instead of caramelizing.
Use warm broth, not cold. Cold broth will cool the pot and interrupt the cooking process; warm broth blends smoothly into the caramelized base.
Don't skip the broiler step or try to melt the cheese on the stovetop—the high, direct heat from the broiler creates that signature bubbly, golden crust that makes this soup special.

Make ahead

Caramelize the onions and make the broth up to 2 days ahead. Store separately in airtight containers in the refrigerator. When ready to serve, reheat the onions and broth in a pot over medium heat until steaming, then proceed with seasoning and serving. The entire soup (without the cheese gratinée topping) can be made up to 1 day ahead and reheated gently on the stovetop.

Storage & reheating

Refrigerator (without cheese topping): 3–4 days in an airtight container. Reheat gently on the stovetop over medium-low heat until steaming. Freezer (without cheese topping): up to 3 months in a freezer-safe container. Thaw overnight in the refrigerator, then reheat. Broiler topping: Cheese is best added fresh just before serving, as it becomes tough if reheated.

Substitutions

Gruyère cheese, finely grated or shredded → Swiss cheese, Emmental, or aged Comté. Any nutty, hard melting cheese works well. Gruyère is traditional but expensive; aged Swiss is an excellent economical substitute with similar flavor.
dry white wine (such as Sauvignon Blanc or Pinot Grigio) → Dry sherry or vegetable broth. Use an equal amount of dry sherry for deeper flavor, or substitute additional broth if you prefer to avoid alcohol. The soup will be slightly less complex but still delicious.
fresh thyme sprigs → 1 teaspoon dried thyme or herbes de Provence. Dried herbs are more concentrated, so use about ¼ the amount. Add them with the broth and fish out any loose pieces before serving, or tie in cheesecloth.
baguette or rustic white bread, sliced into ½-inch-thick rounds → Sourdough, ciabatta, or rustic white bread. Use any crusty bread that holds up to broiling without falling apart. Avoid soft sandwich bread or brioche.