Step 1: Caramelize the onions : In a large heavy-bottomed pot or Dutch oven, heat butter and olive oil over medium heat. Add sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for about 30–40 minutes, until onions are golden brown and caramelized. If the onions start sticking, add a splash of water and scrape the bottom of the pot. Once the onions are deeply golden, stir in garlic and cook for another minute. Add sugar if needed to balance the flavor.

Step 2: Build the base : Sprinkle flour over the onions and stir for 1–2 minutes to create a light roux. Add white wine to deglaze the pot, scraping up any brown bits from the bottom. Simmer until the wine reduces slightly (2–3 minutes).

Step 3: Add broth and herbs : Pour in the beef broth, add thyme and bay leaf, and season with salt and pepper. Bring to a simmer, then reduce heat and let it cook gently for 25–30 minutes. Taste and adjust seasoning as needed. Remove the bay leaf and thyme sprigs before serving.

Step 4: Prepare the bread topping : While the soup simmers, preheat the oven to 400°F (200°C). Place baguette slices on a baking sheet, brush lightly with oil or butter, and toast for 5–7 minutes until golden. Set aside.

Step 5: Assemble and broil : Preheat your oven broiler. Ladle hot soup into oven-safe bowls or crocks. Place 1–2 toasted baguette slices on top of each bowl of soup. Generously sprinkle with grated Gruyère (and Parmesan if using). Place bowls on a baking sheet and broil for 2–4 minutes, until the cheese is bubbly and golden brown.

Step 6: Serve : Carefully remove from oven (the bowls will be hot!) and serve immediately.