French onion soup with Gruyère is a deep brown broth of slowly caramelized onions and beef stock, finished under the broiler with a thick crouton and a blistered layer of cheese. There's no shortcut for the onions; they need about an hour of low, patient heat. The rest is simple, and the whole pot takes about 1 3/4 hours, mostly hands off.
What an hour of caramelizing does to the onions
Properly caramelized onions for soup take about an hour, and trying to speed things up with high heat only scorches the outside while the inside stays sharp. Slowly, the onions give up their water, their sugars brown, and 3 pounds collapse into a jammy tangle. You'll know you're on track by the color and the volume.
| Time | What the onions look like | What to do |
|---|---|---|
| 10 minutes | Softened, glossy, releasing liquid | Lower the heat to medium low |
| 25 minutes | Pale gold, about half the volume | Scrape the bottom every 8 to 10 minutes |
| 45 minutes | Amber, soft and sticky | Add a splash of water if the fond darkens too fast |
| 55 to 65 minutes | Deep mahogany, jammy, a quarter of the start | Ready for the sherry |
Use a heavy-bottomed pot with enough room to stir the onions; adjust the heat if any area browns too quickly. Slice the onions pole to pole, about 1/4 inch thick, so they keep a little shape and the soup has strands you can twirl.
Beef stock, chicken stock and the sherry splash
Rich beef stock is the classic base and gives the darkest, most savory broth. Chicken stock makes a lighter, slightly sweeter soup, and plenty of French cooks use it or blend the two. If you go all chicken, a teaspoon of soy sauce or Worcestershire added with the stock brings back some depth. Whichever you choose, buy low sodium, since the soup reduces and the Gruyère adds salt.
Before the stock goes in, the bottom of the pot carries a dark crust of caramelized sugars. A splash of alcohol dissolves it faster than stock alone, and dry sherry adds a nutty note that sits between the sweet onions and the savory broth. No sherry? These work:
- Dry white vermouth: the closest match, a little more herbal.
- Dry white wine: use 1/2 cup and reduce it by half.
- Brandy or cognac: 2 tablespoons, added off the heat first, then returned to the burner.
- No alcohol: deglaze with 1/3 cup of stock and add 1 teaspoon of sherry vinegar at the end.
Skip bottles labeled cooking sherry, which contain added salt.
Does the soup taste better the next day?
Often, yes, as the onion and beef flavors settle into each other. Make the soup through the simmering step, let it cool, and store it covered in the fridge. The USDA leftovers guidance says to refrigerate cooked food within 2 hours and use it within 3 to 4 days. The base also freezes well for up to 3 months without the bread and cheese.
To serve, bring the soup to a simmer on the stove, taste for salt, and then do the broiler finish with fresh toast and cheese. Don't top it ahead, because the bread turns to paste.
For a winter dinner, it can be the main event with a crisp salad alongside. An autumn harvest salad with apples and a sharp vinaigrette cuts through the rich broth and cheese.
Is Gruyère the only cheese that works?
Gruyère is the classic. It melts smoothly, browns in spots under the broiler and has a nutty, slightly sweet flavor that doesn't get lost next to the onions. Comté is almost identical and a good swap. Emmental melts well but tastes milder, so some cooks blend it half and half with Gruyère. A little grated Parmigiano on top adds extra browning.
Grate the cheese yourself and let it come to room temperature. Cold cheese melts unevenly and can turn rubbery before it bubbles.
The bread should be dry. Day old baguette, sliced about 1 inch thick and toasted until golden, holds its shape when it meets the hot broth. Fresh bread goes soggy and sinks. A crusty country loaf or thick slices of sourdough work too; trim them to fit inside the bowl, since overhanging bread burns before the cheese melts. Plan on about 1/2 cup of cheese per bowl.
Broiling the crocks without burning the cheese
The broiler step is quick, but the bowls get extremely hot and the cheese can go from golden to burnt in under a minute. Set yourself up before you start.
- Use oven safe crocks or ceramic bowls rated for broiler heat. Regular soup bowls can crack.
- Set the filled bowls on a rimmed sheet pan. It catches cheese drips and lets you move all four at once.
- Position the rack so the tops of the bowls sit about 6 inches below the element, and heat the broiler for 5 minutes first.
- Fill the bowls so the soup comes just below the rim, float the toasted bread on top and pile the cheese over it, letting some fall to the edges.
- Broil for 3 to 4 minutes, watching the whole time, until bubbling and spotted brown.
Let the bowls rest for 2 minutes before serving, and set each one on a small plate or folded napkin. They'll stay hot enough to burn for a long while.
The recipe
Classic French Onion Soup with Gruyère Croutons
Ingredients
For the soup
- 5 large yellow onions (about 3 pounds / 1.4 kg), halved and thinly sliced pole to pole
- 3 tablespoons unsalted butter
- 1 tablespoon olive oil
- 1 teaspoon fine sea salt, plus more to taste
- 1/3 cup (80 ml) dry sherry
- 6 cups (1.4 liters) rich beef stock
- 1 bay leaf
- 3 sprigs fresh thyme
- Freshly ground black pepper
For the topping
- 4 slices day old baguette, about 1 inch (2.5 cm) thick
- 2 cups (about 6 ounces / 170 g) grated Gruyère, at room temperature
Instructions
- Soften the onions. Melt the butter with the olive oil in a large Dutch oven over medium heat. Add the onions and salt, stir to coat and cook for 10 minutes, stirring occasionally, until softened and releasing liquid.
- Caramelize slowly. Lower the heat to medium low and cook for 45 to 55 minutes more, stirring and scraping the bottom every 8 to 10 minutes, until the onions are deep mahogany and reduced to about a quarter of their volume. Add a splash of water if the fond threatens to burn.
- Deglaze with sherry. Raise the heat to medium high, pour in the sherry and scrape up every bit of browned fond. Cook for about 2 minutes, until the sherry has nearly evaporated.
- Simmer the soup. Add the beef stock, bay leaf and thyme. Bring to a boil, then lower to a gentle simmer and cook uncovered for 20 to 30 minutes. Remove the bay leaf and thyme stems and season with salt and pepper.
- Toast the bread. Set a rack about 6 inches below the broiler and heat it. Toast the baguette slices on a sheet pan for about 1 minute per side, until lightly golden.
- Top the bowls. Set 4 oven safe bowls on a rimmed sheet pan and ladle in the hot soup. Float a toasted slice on each and pile about 1/2 cup of Gruyère over it, letting some spill to the edges.
- Broil and serve. Broil for 3 to 4 minutes, watching closely, until the cheese is bubbling and spotted golden brown. Let the bowls rest for 2 minutes and serve on plates, warning everyone that the bowls are very hot.
Cook's notes
- The soup base keeps for 3 to 4 days in the fridge or 3 months in the freezer. Add the bread and cheese only when serving.
- Low sodium stock gives you more control, since the soup reduces and the Gruyère adds salt. Season at the end.
- No oven safe bowls? Toast the bread, top it with cheese and broil it on a sheet pan until bubbling, then float the cheesy toasts on the hot soup.
Classic French Onion Soup questions
What are the best onions for French onion soup?
Yellow onions are the best all round choice. They have enough sugar to caramelize deeply and enough sulfur compounds to keep the soup savory rather than sweet. Sweet onions like Vidalia can taste sugary and one note. Some cooks add one red onion or a couple of shallots for extra depth, but yellow onions should make up most of the pot.
Should you add sugar to the onions?
You don't need to. Yellow onions carry enough natural sugar to caramelize deeply given an hour of medium low heat. A pinch of sugar speeds early browning slightly, but it can make the soup taste sweet and flat against the beef stock. Salt matters more, because it draws out water early so the onions soften and color evenly.
Why are my onions burning instead of caramelizing?
The heat is too high or the pot is too thin. Onions need medium low heat for most of the hour so their sugars brown slowly. If the fond on the bottom darkens toward black, add 2 tablespoons of water, scrape it up and lower the heat. Choose a heavy-bottomed pot with enough capacity to stir the onions, and adjust the burner if they catch.
How long does the soup base keep in the freezer?
Up to 3 months. Cool it completely, pack it into airtight containers with some headspace and freeze it without bread or cheese. Thaw it overnight in the fridge, reheat it to a simmer, then add fresh toasted bread and Gruyère and broil as usual. Croutons frozen in the soup turn soggy and fall apart.
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