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Ingredients
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Soup
For Serving / Topping
Method
Soup
- 1
- 2
Caramelize onions
In a large, heavy-bottomed pot or Dutch oven, heat the butter and olive oil over medium heat until the butter melts and foams. Add the sliced onions, sprinkle with sugar and 1 teaspoon of salt, and toss to coat. Cook, stirring every 3–5 minutes, adjusting heat as needed so the onions soften and gradually brown without burning. This process should take 35–45 minutes for deep, even caramelization. If the pan looks dry at any point, add a splash of water or lower the heat.
- 3
Deglaze
When the onions are a rich golden-brown, sprinkle the tablespoon of flour over them and cook for 1–2 minutes, stirring constantly to remove the raw flour taste. Add the white wine to deglaze the pan, scraping up any browned bits from the bottom with a wooden spoon. Let the wine reduce by half, about 2–4 minutes.
- 4
Add brandy and stock
Reduce the heat to medium-low, add the brandy, and simmer for 1 minute to burn off the alcohol. Then pour in the beef stock, add the bay leaf, thyme sprigs, Worcestershire sauce, and the remaining 1/2 teaspoon salt and 1/2 teaspoon pepper (adjust to taste). Bring the soup to a gentle simmer, then lower heat and let it simmer uncovered for 20–30 minutes to meld flavors.
- 5
Finish and adjust seasoning
Remove the bay leaf and thyme sprigs. Taste and adjust seasoning with additional salt, pepper, or a splash more brandy or Worcestershire if desired. Keep the soup warm while you prepare the bread and cheese.
For Serving / Topping
- 6
Toast baguette slices
Preheat your oven broiler (or oven to 425°F/220°C). Arrange baguette slices on a baking sheet, brush lightly with olive oil or butter if desired, and toast under the broiler for 1–2 minutes per side until golden and crisp. Set aside.
- 7
Assemble and gratin
Preheat broiler. Ladle the hot soup into ovenproof bowls (ramekins or crocks). Float 1–2 toasted baguette slices on top of each bowl of soup, then pile a generous amount (about 1/4 cup per bowl) of grated aged Swiss cheese over the bread to cover. Place the filled bowls on a baking sheet and broil 3–6 inches from the heat for 2–4 minutes, watching closely, until the cheese is bubbling and deeply golden-brown. Rotate as needed for even browning.
- 8
