Onions cannot be caramelized in 10 minutes, whatever a recipe card claims. Ten minutes over medium-high heat gives onions that are soft and browned at the edges, which is useful for burgers but nothing like the deep mahogany, jammy onions that give French onion soup its flavor. Properly caramelized onions take about 45 minutes to an hour, and there is no shortcut that produces the same result without trade-offs.
The good news is that most of that hour needs only occasional stirring. Understanding what is happening in the pot, and what the onions should look like at each stage, makes the process predictable rather than tedious. Once the onions are done, the soup itself takes about 20 minutes, and the cheese-topped toast another 5 under the broiler.
The Technique: How to Caramelize Onions
Despite the name, caramelized onions owe their color and flavor to more than one reaction. Onions contain natural sugars, which brown through caramelization at high temperatures, and they also contain amino acids, which react with those sugars through the Maillard reaction at lower temperatures. Both produce brown color and hundreds of new flavor compounds, many of them savory and nutty as well as sweet.
Neither reaction can get going while the onions are full of water. Raw onions are around 90 percent water, and as long as that water is cooking off, the onions stay near 212°F (100°C), well below the temperature where browning speeds up. The first part of caramelizing is therefore really evaporation: the onions shrink, soften and release their liquid. Only once most of it is gone does the browning begin in earnest.
The stages
- Softening, about 10 to 15 minutes: onions wilt, turn translucent and release liquid. The pot looks wet.
- Drying out, about 10 minutes: liquid evaporates, onions shrink to about half their volume and start to turn pale gold.
- Browning, about 20 to 30 minutes: onions move from gold to amber to deep brown. A brown film forms on the bottom of the pot.
- Finished: onions are soft, jammy and the color of strong tea or mahogany, reduced to about a quarter of their raw volume.
Deglazing as you go
During the browning stage, a layer of fond builds on the bottom of the pot. When it turns a deep brown, add 2 or 3 tablespoons of water and scrape it up with a wooden spoon. The water dissolves the fond and coats the onions with it, and it cools the pot slightly so nothing burns. Repeat each time the fond darkens. This is the single most useful technique for deep, even color.
Why the Shortcuts Fall Short
High heat browns onions faster, but unevenly. The pieces touching the pot scorch while those on top are still pale, and scorched onion tastes bitter. Medium heat for the first stage and medium-low for the browning stage is the most reliable approach.
A pinch of baking soda does speed browning. It raises the pH of the onions, and the Maillard reaction runs faster in alkaline conditions. The trade-off is texture: the onions break down into a soft paste and can taste faintly soapy if too much is used. For soup, where texture matters, the slower method gives better results. If time is very short, use no more than 1/8 teaspoon for 4 pounds of onions.
A slow cooker can soften onions overnight with no attention, but the closed lid traps moisture, so they come out pale and wet. They still need 20 to 30 minutes in an open pot on the stove to brown properly.
Applying It to French Onion Soup
Four pounds of onions sounds like a great deal. It cooks down to about 2 cups, which is the right amount for six bowls of soup. Yellow onions are the best choice: they have a good balance of sugar and sharpness. Sweet onions, such as Vidalia, brown readily but can make the soup taste sugary; red onions turn a muddy color.
Slice the onions from root to tip, pole to pole, into strips about 1/4 inch thick. Slices cut this way hold their shape better during the long cook than rings cut across the onion, which tend to collapse into mush. A wide, heavy pot, such as a 5 1/2- to 7-quart Dutch oven, gives the onions more surface contact, which helps the water evaporate faster.
The broth
Classic French onion soup uses beef stock, which gives depth and color. Choose a low-sodium version, because the stock simmers and reduces slightly. A mix of beef and chicken stock gives a lighter, still rich broth. A splash of dry sherry or white wine, used to deglaze the onions once they are done, adds acidity that keeps all that sweetness from feeling heavy.
The Gruyère toast topping
Slices of baguette are toasted until completely dry and crisp, so they float on the soup without dissolving instantly. Grated Gruyère goes on top and melts under the broiler. Gruyère melts smoothly and browns well, and its nutty flavor suits the onions. Comté or Emmental are good substitutes.
Correcting Course While the Onions Cook
An hour is long enough for small problems to appear, and nearly all of them can be fixed in the pot.
Onions still pale after 40 minutes
This usually means the pot is crowded or the heat is too low for the water to evaporate. Turn the heat up to medium for 5 to 10 minutes, stirring more often, until the onions look dry and the bottom of the pot starts to brown, then turn it back down. A narrow, tall pot slows evaporation; next time, use a wider one or cook in two batches.
Dark spots and a sharp smell
A few onions stuck to the bottom have burned. Pick out any that are black, add 1/4 cup of water, and scrape the pot thoroughly. Lower the heat and stir more often. As long as the overall smell is sweet and nutty, the batch is fine.
Onions turning to mush
Slices cut very thin, or across the onion into rings, can collapse into a paste. Stir more gently, folding rather than churning, and next time slice pole to pole at 1/4 inch. Mushy onions still taste good in the soup; the texture is just less distinct.
Soup tastes too sweet
Very sweet onions or long cooking can push the soup toward sugary. Add a teaspoon of sherry vinegar or red wine vinegar, a pinch more salt, and taste again. Acid and salt both balance sweetness.
Make the French Onion Soup
Serves six. You need oven-safe bowls or crocks for the broiler step; set them on a sheet pan to make moving them easier and to catch any spills.
The recipe
French Onion Soup
Four pounds of yellow onions slowly caramelized for about an hour, simmered in beef stock with sherry and thyme, topped with toasted baguette and broiled Gruyère.
- Prep
- 20 min
- Cook
- 1 hr 35 min
- Total
- 1 hr 55 min
- Serves
- 6
Ingredients
Method
- Soften. Melt the butter in a 5 1/2- to 7-quart Dutch oven over medium heat. Add the onions and salt and cook, stirring every few minutes, for 20 to 25 minutes, until soft, shrunken by about half and starting to turn pale gold.
- Caramelize. Lower the heat to medium-low. Cook 25 to 35 minutes more, stirring every 3 to 5 minutes. Each time a deep brown film forms on the bottom, add 2 to 3 Tbsp water and scrape it up. Stop when the onions are a deep mahogany color and jammy.
- Deglaze. Add the sherry and scrape the pot clean. Simmer until almost evaporated, about 2 minutes.
- Simmer. Add the stock, thyme, bay leaf and pepper. Simmer gently, partly covered, for 20 minutes. Remove the thyme stems and bay leaf and season to taste with salt.
- Toast the bread. Meanwhile, heat the oven to 400°F (205°C). Bake the baguette slices on a sheet pan for 10 to 12 minutes, until dry and crisp throughout.
- Broil. Heat the broiler. Set six oven-safe crocks on a sheet pan, ladle in the hot soup, top each with 2 toasts and a generous handful of Gruyère. Broil 6 inches from the element for 3 to 5 minutes, until bubbling and browned in spots. Rest 5 minutes before serving; the crocks are very hot.
Notes
- Caramelized onions keep 5 days refrigerated or 3 months frozen.
- The soup without topping keeps 3 to 4 days refrigerated; reheat to a simmer before broiling.
Oven temperatures are for conventional ovens; for convection, reduce by about 25°F (15°C).
Making It Ahead
Caramelized onions keep for up to 5 days in the refrigerator and freeze well for about 3 months. Cooking a double batch on a quiet day and freezing half in 2-cup portions turns French onion soup into a 30-minute dinner later. The finished soup, without bread and cheese, also keeps for 3 to 4 days refrigerated and about 3 months frozen.
Reheat the soup to a simmer before ladling it into the crocks; the broiler only melts the cheese and does not heat the soup through. Toast the bread and grate the cheese fresh each time.