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Ingredients
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Soup Base
Croutons & Topping
Method
Soup Base
Prepare onions
Peel and thinly slice onions into half-moons so you have about 6 cups loosely packed. Patience with slicing yields even caramelization.
Caramelize onions
In a large heavy-bottomed pot or Dutch oven, heat 3 tbs unsalted butter and 1 tbs olive oil over medium heat until butter foams. Add all sliced onions and 1 tsp granulated sugar, stir to coat. Cook, stirring frequently, for 30–40 minutes, reducing heat as needed to maintain a gentle deep golden-brown (not burned). Scrape browned bits from the bottom occasionally. Total time depends on stovetop; this step builds the soup’s flavor.
Deglaze
When onions are deeply browned, sprinkle 1 tbs flour over them and cook 1–2 minutes, stirring to remove raw flour taste. Pour in 1 cup dry white wine to deglaze, scraping the pot to release browned bits. Let the wine reduce for 2–3 minutes.
Simmer with stock
Add 6 cups beef stock, 1 bay leaf, 2 thyme sprigs, 1 tsp Worcestershire sauce, 1.5 tsp salt, and 1/2 tsp black pepper. Bring to a gentle simmer, then reduce heat and simmer uncovered for 20–30 minutes to meld flavors. Taste and adjust seasoning; remove bay leaf and thyme stems before serving.
Croutons & Topping
Toast baguette slices
Preheat oven to 400°F (205°C). Arrange 1/2-inch baguette slices on a baking sheet, brush both sides lightly with 2 tbs olive oil or melted butter. Toast in the oven 6–8 minutes, flipping halfway, until crisp and lightly golden. Set aside.
Assemble and broil
Preheat the oven broiler. Ladle the hot soup into oven-safe bowls (leave about 1 inch from top). Place 1–2 toasted baguette slices on top of each bowl of soup (or float the croutons), then generously pile grated Gruyère (about 1/3–1/2 cup per bowl) and a sprinkle of Parmesan if using. Place bowls on a baking sheet and broil 2–4 minutes, watching closely, until cheese is bubbly and golden brown.
Serve
Carefully remove hot bowls from oven (use oven mitts). Let rest 1–2 minutes, then serve immediately while cheese is molten. Garnish with a small thyme leaf if desired.