Onion Soup Recipe

Prepare onions

Peel the onions, cut off ends, and slice them thinly into half-moons so slices are about 1/8-inch thick. Aim for uniform slices so they cook evenly.

Melt butter and oil

In a large heavy-bottomed pot or Dutch oven over medium heat, add the butter and olive oil. Heat until the butter is melted and begins to foam but not brown.

Cook onions slowly

Add the sliced onions, sprinkle the kosher salt and sugar over them. Stir to coat. Cook over medium-low to medium heat, stirring frequently, for 30-40 minutes until the onions are deeply golden brown and caramelized. If onions start to stick, reduce heat slightly and deglaze with a tablespoon of stock or water, scraping up brown bits.

Season and add flour

When onions are richly caramelized, stir in the black pepper. Sprinkle the tablespoon of flour over the onions and cook, stirring, 1-2 minutes to remove the raw flour taste and help thicken the soup slightly.

Deglaze with wine

Pour in the dry white wine (or sherry), increase heat to medium-high briefly, and scrape up any browned bits from the bottom of the pot. Let the wine reduce for 1-2 minutes until mostly evaporated.

Add stock and aromatics

Add the beef stock, bay leaves, thyme sprigs, and Worcestershire sauce. Stir to combine and bring to a gentle simmer.

Simmer

Reduce heat to low and simmer, uncovered, for 20-25 minutes to let flavors meld. Taste and adjust seasoning with additional salt and pepper if needed. Remove and discard bay leaves and thyme stems.

Toast baguette slices

Preheat oven broiler or set oven to 425°F (220°C). Arrange baguette slices on a baking sheet, brush lightly with the 1 tablespoon olive oil, and toast under the broiler or in the oven until golden and crisp, about 1-2 minutes per side under broiler or 6-8 minutes at 425°F, turning once. Remove and set aside.

Assemble and broil (individual bowls)

Preheat broiler. Ladle the hot soup into ovenproof bowls or crocks, leaving about 1/2-inch headspace. Place one or two toasted baguette slices on top of each bowl and generously sprinkle with Gruyère (and a little Parmesan if using). Place bowls on a baking sheet and broil 2-4 inches from the heat until cheese is melted, bubbly, and lightly browned, 2-5 minutes (watch closely to avoid burning). Carefully remove from oven and let cool a minute before serving.

Assemble (for a single casserole-style serving)

Alternatively, pour soup into a shallow ovenproof dish, arrange toasted baguette slices on top, cover with cheese, and broil until bubbly and browned. Let rest 1-2 minutes before serving.

Serve

Serve hot, taking care that bowls are hot and cheese is bubbling. Garnish with a small thyme leaf if desired.