1. Slice and caramelize the onions: In a large heavy-bottomed pot, heat butter and olive oil over medium heat. Add the sliced onions with a pinch of salt. Cook, stirring occasionally, for 40–45 minutes until they’re deep golden brown and caramelized.

2. Add garlic and deglaze: Stir in the minced garlic and cook for another minute. Pour in white wine to deglaze the pan, scraping up any browned bits. Simmer for about 5 minutes.

3. Simmer the soup: Add beef broth, bay leaf, thyme, black pepper, and bring to a simmer. Let it cook uncovered for 20–25 minutes. Taste and adjust seasoning with salt as needed.

4. Toast the baguette: While the soup simmers, toast baguette slices under the broiler until crisp and golden.

5. Assemble and broil: Preheat your broiler. Ladle hot soup into oven-safe bowls. Top each with a slice of toasted bread and a generous layer of Gruyère and Parmesan. Broil until cheese is bubbly and lightly browned, about 3–5 minutes.

6. Serve hot: Garnish with extra thyme if desired. Serve carefully—bowls will be hot!