French Onion Recipe

Prepare onions

Peel and halve the onions, then cut each half into thin, even slices (about 1/8-inch). Aim for uniform slices so they caramelize evenly.

Caramelize onions

In a large heavy-bottomed pot or Dutch oven, heat the butter and 1 tablespoon olive oil over medium heat until butter melts. Add the sliced onions, spread them evenly, and sprinkle with 1 teaspoon kosher salt and 1 teaspoon granulated sugar. Stir to coat. Cook the onions, stirring every 5–7 minutes, lowering heat as needed to maintain a gentle steady sizzle. This process takes 35–45 minutes: onions should become deeply golden brown and jammy. Scrape browned bits from the pot sides with a wooden spoon to incorporate.

Deglaze

When onions are richly caramelized, sprinkle 1 tablespoon flour over them and stir for 1–2 minutes to cook the raw flour taste (this helps slightly thicken the soup). Add the dry white wine (or 1/2 cup extra stock if omitting wine) and stir, scraping the bottom to release any fond. Let the wine reduce for 2–3 minutes.

Simmer the broth

Add 6 cups beef stock and 1 cup chicken stock (if using), the thyme sprigs, and bay leaves. Bring to a gentle simmer, then lower heat and simmer uncovered for 20–25 minutes to meld flavors. Skim any foam or excess fat from the surface. Stir in 2 tablespoons sherry or demi-glace if using. Taste and season with additional salt and freshly ground black pepper as needed.

Finish

Remove and discard thyme sprigs and bay leaves. Keep the soup hot while you prepare the toasts and cheese topping. If soup is too thin, simmer a few minutes longer; if too concentrated, add a splash of stock or water and re-taste.

Prepare toasts

Preheat the oven broiler or set oven to 450°F (230°C) on the top rack. Arrange baguette slices on a baking sheet. Brush both sides lightly with 1 tablespoon olive oil. Toast under the broiler or in the oven until edges are golden, about 1–2 minutes per side; watch carefully to avoid burning.

Assemble bowls

Ladle the hot onion soup into oven-safe bowls or crocks, filling them about 3/4 full. Place one or two toasted baguette slices on top of each bowl, then mound generously with grated Gruyère and a sprinkle of Parmesan if using. Cheese should cover the bread and slightly overlap the rim to form a gratin.

Gratinate

Place the bowls on a baking sheet to catch drips and broil until the cheese is bubbly and deeply golden brown in spots, about 3–6 minutes depending on broiler strength. Rotate pans if needed for even browning. Remove carefully (use oven mitts) and let cool 2–3 minutes before serving — cheese and bowls will be very hot.