Thinly slice onions and mince garlic. Measure wine and broth. Grate Gruyère and set out oven-safe bowls.

Heat butter and olive oil in a heavy-bottomed pot over medium heat. Add onions and sprinkle with sugar. Cook slowly, stirring occasionally, until deeply browned and sweet (about 30 minutes). Adjust heat to prevent burning.

Stir in garlic and cook 1 minute. Add white wine and scrape the bottom to release browned bits. Simmer until the wine reduces by about half.

Add beef broth, bay leaf, and thyme. Bring to a simmer, then reduce heat to low and partially cover. Simmer gently for 30 minutes to deepen flavor.

While soup simmers, toast baguette slices at 350°F (175°C) until golden, about 10 minutes.

Taste and season soup with salt and black pepper. Keep in mind the cheese adds saltiness.

Preheat broiler. Ladle soup into oven-safe bowls. Top each with a toasted baguette slice and a generous layer of grated Gruyère.

Broil 3–5 minutes until cheese is melted, bubbly, and golden brown. Watch closely to prevent burning. Serve immediately.