Classic French Onion Soup

Notes

The defining step in French onion soup is properly caramelizing the onions. This cannot be rushed. The onions should be deep mahogany, not merely soft or golden, before the wine and stock are added. Four pounds of raw onions will reduce to a surprisingly small, intensely flavored mass.

Slice the onions from pole to pole rather than across the equator. This follows their natural fibers and helps them retain enough structure during the long cooking process. Slices approximately 1/4 inch thick caramelize evenly without disappearing into paste.

No sugar is needed. Patient cooking develops the onions' natural sweetness. Small additions of water help dissolve the browned fond and prevent scorching without interrupting caramelization.

The soup is only as good as its stock. Use unsalted homemade stock or high-quality refrigerated stock whenever possible. Many shelf-stable beef broths taste metallic or excessively salty after simmering.

The chicken stock softens the sometimes heavy flavor of all-beef stock while allowing the soup to remain clearly beef-based.

For the classic presentation, use only bowls specifically labeled broiler-safe. If suitable crocks are unavailable, broil the cheese-covered toasts separately on a sheet pan and float them on each bowl immediately before serving.

Substitutions

A combination of yellow and sweet onions may be used, but avoid using only very sweet onions because the finished soup can become cloying. Red onions will alter the color and flavor.

Dry vermouth may replace the white wine. Madeira or additional dry sherry may be used for a richer variation. Never use salted cooking wine or cooking sherry.

Comté, Emmental or a blend of Gruyère and provolone may replace Gruyère. Mozzarella alone lacks the characteristic nutty flavor.

A sturdy country loaf may replace baguette if cut to fit the crocks and toasted thoroughly.

For an alcohol-free version, replace the wine and sherry with additional unsalted stock and finish the broth with 1–2 teaspoons sherry vinegar. The flavor will differ from the original.

Variations

Cognac French Onion Soup:
Add 2 tablespoons Cognac after the wine has reduced and simmer for 1–2 minutes before adding the stock.

Three-Onion Soup:
Replace 1 pound of the yellow onions with sweet onions and add 2 thinly sliced leeks after the yellow onions have cooked for 20 minutes.

Short Rib French Onion Soup:
Add 2 cups shredded braised short rib to the finished broth before assembling the crocks. Reduce the salt because braised meat and its juices may be highly seasoned.

Individual Gruyère Croutons:
When broiler-safe crocks are unavailable, pile cheese on the toasted baguette slices and broil them separately. Float the finished croutons on bowls of hot soup just before serving.

Storage

Prepare the soup base through the final seasoning, cool it promptly and refrigerate in shallow airtight containers for up to four days. Store the toasted baguette and grated cheese separately.

The soup base freezes well for up to three months. Thaw overnight in the refrigerator and prepare fresh toast and cheese topping when serving.

Reheating

Reheat the soup base in a saucepan over medium heat, stirring occasionally, until it reaches a gentle simmer and is thoroughly hot. Taste and adjust the seasoning after reheating.

Assemble each serving with freshly toasted baguette and grated cheese, then broil as directed. Do not store or reheat assembled crocks because the bread becomes soggy and the cheese can turn greasy.

Troubleshooting

The onions are pale after an hour:
The heat may be too low, the pot may be too narrow or the onions may still be releasing moisture. Continue cooking uncovered and stir more frequently as they darken.

The onions are burning:
Reduce the heat immediately and add 1–2 tablespoons water while scraping the pot. Blackened, bitter onions cannot be repaired and should not be incorporated into the soup.

The soup tastes sweet but lacks depth:
The onions may have softened without browning sufficiently, or the stock may be weak. Continue caramelizing until mahogany and use a concentrated, high-quality stock.

The soup is too salty:
Use unsalted stocks and season only after reduction. Dilute an overly salty batch with unsalted chicken stock or water. Remember that the cheese topping adds more salt.

The broth tastes harshly alcoholic:
The wine and sherry were not reduced sufficiently. Simmer them until reduced by about half before adding the stock.

The bread dissolved immediately:
It was not toasted thoroughly. Dry the baguette until crisp on both sides before floating it on the soup.

The cheese is greasy rather than browned:
Use freshly grated Gruyère, keep the broiler fully preheated and place the crocks close enough to the element for quick browning.

The cheese topping slid into the soup:
Use two overlapping toast slices to cover most of the surface and allow the cheese to extend to the rim of the crock.