Heat a large skillet over medium heat and add butter and olive oil. Once melted, add the sliced onions, thyme, and bay leaf. Cook on low for about 45 minutes, stirring every 10 minutes, until caramelized.

Remove the bay leaf, pour in the white wine to deglaze, and scrape up bits from the skillet. Cook for 2-3 minutes, then add the beef broth. Bring to a gentle boil, reduce to low, and simmer uncovered for 15 minutes.

Preheat the broiler. Ladle the soup into oven-safe bowls, filling them three-quarters full. Add toasted baguette slices on top, then sprinkle with grated Gruyère cheese.

Broil the bowls for 3-5 minutes until the cheese bubbles and browns. Watch closely to avoid burning.

Garnish with fresh thyme leaves and serve immediately for a comforting dining experience.