01 -
Heat butter and olive oil in a large high-sided skillet over medium heat. Add sliced onions, thyme leaves stripped from sprigs, and bay leaf. Cook on low heat, stirring every 10 minutes to prevent sticking, until onions are deeply caramelized and golden brown, about 45 minutes. Season with salt during cooking.
02 -
Increase heat and deglaze the pan with white wine, boiling until alcohol evaporates. Add beef broth and return to a boil. Reduce heat and simmer gently while broiler preheats. Season to taste with salt and pepper.
03 -
Ladle soup evenly into four oven-safe bowls. Top each with a toasted baguette slice and divide grated Gruyere cheese evenly on top.
04 -
Place bowls under preheated broiler. Broil until cheese melts, bubbles, and turns golden, approximately 3 to 5 minutes. Watch closely to prevent burning.
05 -
Add fresh thyme leaves if desired and serve immediately for best flavor and texture.