In a large, heavy-bottomed pot, melt the butter over medium-low heat. Add the thinly sliced onions and stir to coat. Cook, stirring occasionally for about 30-40 minutes until deep golden brown.

Add minced garlic and cook for an additional 2 minutes until fragrant.

Pour in white wine, scraping the bottom of the pot. Increase heat to medium and simmer for about 5 minutes.

Reduce heat to low and add beef broth, thyme, and bay leaves. Simmer gently for 30 minutes.

Taste and season with salt and pepper as needed.

Preheat oven to 400°F (200°C). Arrange bread on a baking sheet and toast for 5-7 minutes.

Ladle soup into oven-safe bowls. Place toasted bread on top, sprinkle with Gruyère and Parmesan cheese, and broil for 3-5 minutes until bubbly.

Remove from oven, let sit briefly, and serve warm.