Set a large heavy pot over medium-low heat. Add butter and olive oil; once melted, add sliced onions and a pinch of salt. Stir to coat.

Cook onions low and slow, stirring occasionally, for about 10 minutes. Sprinkle in sugar to encourage even browning. Continue cooking 30–40 minutes, stirring frequently, until deep golden and jammy. If browning too fast, lower heat and add a splash of water to deglaze.

Stir in minced garlic and cook 30–60 seconds until fragrant. Add Worcestershire and scrape up browned bits from the pot.

Pour in broth; add thyme, black pepper, salt, and bay leaf. Bring to a gentle simmer and cook 20–25 minutes to meld flavors. Taste and adjust seasoning.

Meanwhile, preheat oven to 375°F (190°C). Arrange baguette slices on a baking sheet and toast 8–10 minutes, flipping halfway, until crisp and golden.

Ladle hot soup into oven-safe bowls on a baking sheet. Top each with a toasted baguette slice and a generous handful of shredded Gruyère.

Switch oven to broil. Broil 3–5 minutes, watching closely, until cheese is melted, bubbly, and lightly browned.

Carefully remove bowls. Let cool briefly before serving. Discard bay leaf and enjoy.