Thinly slice the onions and melt butter with olive oil in a pot over medium-low heat. Add onions and cook for 25-30 minutes, stirring occasionally until caramelized.

Sprinkle salt and thyme, stir well, then add beef broth and bay leaf. Increase heat to medium-high and simmer uncovered for 15 minutes.

Taste the soup and adjust seasoning with black pepper and salt as needed. Remove bay leaf and simmer for another 5 minutes.

Preheat broiler. Toast baguette slices until golden, about 2-3 minutes.

Ladle soup into bowls, top with toasted baguette, sprinkle with Gruyère and Parmesan cheese, and broil until cheese is melted and bubbly, about 2-3 minutes.

Remove from oven, let sit for a minute, garnish with fresh thyme, and serve.