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Ingredients
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Soup Base
For Serving / Topping
Method
Soup Base
Prepare onions
Peel and thinly slice onions into even 1/8- to 1/4-inch slices. Aim for uniformity so they cook evenly.
Caramelize onions
In a large heavy-bottomed pot or Dutch oven, heat butter and olive oil over medium heat until butter foams. Add sliced onions, sprinkle with salt and granulated sugar. Stir to coat. Cook, stirring frequently, for 20–30 minutes until onions are deeply golden and caramelized. If they start to stick, reduce heat slightly and scrape browned bits from the bottom with a wooden spoon. Adjust heat as needed to prevent burning; patience yields flavor.
Deglaze
Raise heat to medium-high and pour in the white wine (if using). Scrape the bottom of the pot to release all brown fond. Cook until most of the wine has evaporated, about 2–3 minutes. If not using wine, add a small ladle (1/4 cup) of stock to deglaze instead.
Simmer the broth
Add beef stock, chicken stock, and beef bouillon paste (if using) to the pot with the caramelized onions. Add thyme sprigs and bay leaf. Bring to a gentle simmer, then reduce heat to low and simmer uncovered for 25–30 minutes to meld flavors. Taste and season with additional salt and freshly ground black pepper as needed. Remove thyme sprigs and bay leaf before serving.
For Serving / Topping
Toast baguette slices
While the soup simmers, preheat oven to 400°F (200°C) or use a broiler. Brush both sides of the baguette slices lightly with olive oil. Place slices on a baking sheet and toast in the oven until golden and crisp, about 6–8 minutes, flipping once. If broiling, watch closely and broil 1–2 minutes per side. Optionally rub the hot toasted slices with the cut side of the garlic clove for a subtle garlic flavor.
Assemble and melt cheese
Ladle hot soup into oven-safe bowls (leave a 1/2-inch rim). Place one or two toasted baguette slices on top of each bowl of soup. Sprinkle 1/4 to 1/2 cup grated Gruyère over the bread, finishing with a light dusting of Parmesan if using. Place bowls on a baking sheet and transfer to the oven under the broiler (or at 450°F/230°C) until cheese is bubbly and browned in spots, about 2–5 minutes. Watch carefully to avoid burning.
Serve
Carefully remove bowls from oven using oven mitts. Let sit 1–2 minutes to settle, then serve immediately. Garnish with a small thyme sprig if desired.