Classic French Onion Soup with Gruyère Toasts

Directions

1. Slice the onions consistently

   Halve the onions through the root, peel them and slice lengthwise into even 1/4-inch arcs. Consistent slices soften and brown at the same rate; very thin pieces burn before thicker ones caramelize.

2. Start the onions covered

   Melt the butter with 1 tablespoon olive oil in a wide 6-quart Dutch oven over medium heat. Add the onions, 1 teaspoon salt and sugar. Cover and cook for 15 minutes, stirring twice, until the onions collapse and release substantial liquid.

3. Caramelize patiently

   Uncover, reduce the heat to medium-low and cook for 35 to 45 minutes, stirring every 5 minutes and more frequently near the end. Scrape the brown film from the pot into the onions. They are ready when uniformly deep chestnut-brown, jammy and reduced to about 3 cups; lower the heat if the fond turns black.

4. Build the savory base

   Stir in the garlic for 30 seconds, then sprinkle over the flour and cook for 1 minute. Add the sherry and scrape the bottom until the browned bits dissolve, about 2 minutes. Let the alcohol aroma soften before adding broth.

5. Simmer the soup

   Add the beef broth, thyme, bay leaf, pepper and Worcestershire. Bring to a boil, then reduce to a gentle uncovered simmer for 25 minutes. Remove the thyme stems and bay leaf. Taste and add the remaining 1/2 teaspoon salt only as needed because broth and cheese vary in salinity.

6. Toast the baguette

   Heat the oven to 425°F. Arrange the baguette slices on a rimmed sheet pan, brush both sides with 2 teaspoons olive oil and bake for 5 to 7 minutes, turning once, until dry and golden at the edges.

7. Prepare a safe broiling station

   Set six broiler-safe 12- to 14-ounce crocks on a foil-lined rimmed sheet pan. Verify that the bowls are explicitly rated for broiler heat; ordinary ceramic can crack. Move the oven rack so the cheese will sit about 6 inches below the element and heat the broiler to high.

8. Cap the soup with Gruyère

   Divide the hot soup among the crocks, leaving 1/2 inch at the top. Float two toasted baguette slices on each serving and cover with Gruyère and Parmesan all the way to the edges, forming the continuous cheese cap visible on the finished soup.

9. Broil and serve carefully

   Broil for 2 to 4 minutes, watching continuously, until the cheese bubbles and develops deep golden patches. Remove the entire sheet pan with heat-safe mitts, scatter with chives and rest for 5 minutes. Warn diners that both crocks and soup remain extremely hot.

Why this works

Swaps & notes

Easy swaps

• Dry white wine can replace sherry. For an alcohol-free version, use 1/2 cup additional broth plus 2 teaspoons sherry vinegar added after simmering.
• Emmental, Comté or a mixture of Swiss cheese and Gruyère can replace all or part of the Gruyère while preserving good melting.
• For vegetarian soup, use a robust mushroom broth and vegetarian Worcestershire sauce; the flavor will be earthier and slightly lighter.

Cook notes

• Three pounds of onions look excessive at first but cook down dramatically. A wide pot is important because a deep crowded layer steams for too long.
• If the pot dries before the onions are deeply brown, add 1 tablespoon water and scrape up the fond. Repeating this is safer than raising the heat.
• Broiler-safe crocks are essential. If yours are only oven-safe, melt the cheese on the baguette slices separately and place the finished toasts over hot soup.

Storage

Cool the soup without bread or cheese and refrigerate in shallow airtight containers within 2 hours for up to 4 days. Refrigerate toasted baguette separately for 2 days. Reheat soup to a full simmer and at least 165°F, then assemble and broil fresh portions. Freeze the onion-broth base for up to 3 months; add new toast and cheese after thawing.

Questions