Caramelize the onions:
In a large pot, melt butter with olive oil over medium-low heat.
Add onions, sugar, and salt. Cook slowly, stirring often, for 25–30 minutes until onions are golden brown and caramelized.
Add garlic & deglaze:
Stir in garlic and cook for 1 minute.
Add wine (if using) and scrape the pot to loosen any browned bits. Simmer 2–3 minutes.
Build the broth:
Add beef broth, thyme, bay leaf, and pepper.
Simmer uncovered for 20–25 minutes. Remove bay leaf. Taste and adjust seasoning.
Toast the bread:
While soup simmers, toast baguette slices in the oven until crisp.
Assemble & serve:
Ladle soup into oven-safe bowls. Top with 1–2 toasted baguette slices. Sprinkle generously with Gruyère cheese.
Place bowls under the broiler until cheese melts and turns golden-bubbly.
Enjoy:
Serve hot, with extra bread for dipping!
Tips:
If you don’t have Gruyère, Swiss or provolone also work well.
The key to flavor is patience with the onions—don’t rush the caramelization.
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