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Ingredients
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Soup Base
Topping & Garnish
Method
Soup Base
Prepare onions
Peel and cut yellow onions in half lengthwise, then thinly slice crosswise to yield about 6 cups. Keeping slices even will help them caramelize uniformly.
Sweat and caramelize
In a large heavy-bottomed pot or Dutch oven, heat 1 tablespoon olive oil and 3 tablespoons unsalted butter over medium heat until butter foams. Add all sliced onions and toss to coat. Cook, stirring every 2–3 minutes, for about 20–30 minutes until onions are deep golden brown. If onions begin to stick or brown too fast, reduce heat to medium-low. Sprinkle 1 teaspoon granulated sugar over onions after 10 minutes to encourage even caramelization.
Deglaze
Once onions are richly browned, sprinkle 1 tablespoon flour over them and cook 1 minute, stirring, to remove raw flour taste. Pour in 1 cup dry white wine (or additional stock if omitting wine) and scrape the bottom of the pot with a wooden spoon to release browned bits. Let the wine reduce by half, about 2–3 minutes.
Simmer soup
Add 6 cups beef stock, 1 bay leaf, 2 tablespoons fresh thyme (or 2 tsp dried), 1 teaspoon Worcestershire sauce, 1 teaspoon salt, and 1 teaspoon freshly ground black pepper. Bring to a gentle simmer, then reduce heat to low. Cover partially and simmer for 20 minutes to allow flavors to marry. Taste and adjust seasoning with additional salt and pepper as needed. Remove and discard bay leaf and thyme stems (if used whole sprigs).
Topping & Finish
Prepare crostini
Preheat broiler (or oven to 425°F / 220°C). Arrange 1/2-inch baguette slices on a baking sheet. Brush both sides lightly with 1 tablespoon olive oil (or spread a thin smear of softened butter). Toast under broiler or in oven until edges are golden, about 1–2 minutes per side; watch carefully to avoid burning.
Assemble and gratinate
Ladle soup into ovenproof bowls or crocks, leaving about 1/2-inch headspace. Place one or two toasted baguette slices on top of each bowl, then pile generously with grated Gruyère (about 1/4 cup per bowl, more if desired). Place bowls on a baking sheet and broil until cheese is fully melted and bubbly with golden brown spots, 2–4 minutes. If you prefer, melt cheese under 425°F oven for 6–8 minutes instead of broiling.
Garnish and serve
Carefully remove hot bowls from oven with oven mitts. Sprinkle each bowl with a pinch of chopped fresh parsley for color. Serve immediately with an extra crostini on the side if desired.