French Onion Recipe

Prepare onions

Peel the yellow onions, cut in half through the root, and thinly slice each half to yield even 1/8- to 1/16-inch slices. Separate slices into strands and set aside.

Caramelize onions

In a large heavy-bottomed pot or Dutch oven, warm the butter and olive oil over medium heat until butter foams. Add the sliced onions, kosher salt, and granulated sugar (the sugar helps caramelization). Stir to coat and spread onions in an even layer. Cook over medium to medium-low heat, stirring every 3–5 minutes to prevent burning. After 20–25 minutes the onions will soften and begin to brown; continue cooking another 15–20 minutes until deep golden-brown and richly caramelized. If the pan scorches, lower the heat and deglaze early with a splash of stock or water.

Add flour and deglaze

Once onions are deeply caramelized, sprinkle the tablespoon of all-purpose flour over them and stir for 1–2 minutes to cook out the raw flour taste and help thicken the broth slightly. Pour in the 1/2 cup dry white wine (or dry vermouth) to deglaze the pot, scraping up browned bits from the bottom with a wooden spoon. Let the wine reduce by about half, 2–3 minutes.

Simmer soup

Add the 6 cups beef stock, bay leaf, and thyme sprigs. Increase heat to bring the soup to a gentle boil, then lower to a simmer. Simmer, uncovered, for 20–25 minutes to allow flavors to meld. Skim any foam or excess fat from the surface. Remove and discard the bay leaf and thyme stems. Season with 1/2 teaspoon freshly ground black pepper and additional kosher salt to taste (start with 1/4–1/2 teaspoon and adjust).

Toast baguette slices

Preheat the broiler, or a 425°F (220°C) oven if broiler is not available. Arrange baguette slices on a baking sheet and brush one side lightly with olive oil or softened butter. Toast under the broiler or in the oven until golden and crisp, about 1–2 minutes per side under a broiler or 6–8 minutes at 425°F, watching closely to avoid burning. Remove and set aside.

Assemble croûtes

Mix grated Gruyère and grated Parmesan in a bowl. Pile a generous tablespoon of the cheese mixture on each toasted baguette slice. Place the cheese-topped slices back on the baking sheet.

Finish under broiler

When the soup is ready and ladled into ovenproof bowls, float one or two cheese-topped croûtes on each bowl of soup (or place the bowls on a baking sheet and top each with cheese and bread). Place bowls on a baking sheet and broil until the cheese is bubbly and deeply golden, about 2–4 minutes depending on broiler strength. Rotate as needed for even melting. Remove carefully; cheese will be very hot. Grind a little extra black pepper over the top to finish.