Melt butter in a large pot over medium heat.
Add onions, cooking until caramelized (about 40 minutes), stirring frequently.
Incorporate garlic and cook for an additional minute.
Deglaze with white wine, scraping the pot’s bottom.
Add beef broth, bay leaves, and thyme. Season with salt and pepper.
Bring to a boil, then simmer for 30 minutes.
Under a preheated broiler, place ladled soup in oven-safe bowls, topped with baguette and Gruyère.
Broil until the cheese is bubbly and golden.
Tips, Shortcuts, and Variations
Shortcut: Use a mandoline for even, thin onion slices.
Variation: For a vegetarian twist, swap beef broth with a rich vegetable broth.
Tip: For best results, use homemade beef broth.
Swap: If Gruyère is unavailable, try Emmental or a mature Cheddar.
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