In a large pot, heat the olive oil and butter over medium heat. Add the sliced onions and cook, stirring occasionally, until they turn golden brown and caramelized, about 40 minutes.
Add Garlic And Seasonings
Stir in the minced garlic and cook for an additional 2 minutes until fragrant. Add the thyme leaves, bay leaf, salt, and pepper.
Deglaze And Simmer
Pour in the white wine (if using) and let it simmer for 5 minutes, allowing the alcohol to evaporate. Add the beef broth, bring the mixture to a boil, then reduce the heat and let it simmer uncovered for 20 minutes.
Prepare The Bread
While the soup simmers, preheat the oven to 400°F (200°C). Place the bread slices on a baking sheet and toast them in the oven until golden brown, about 10 minutes.
Assemble And Broil
Ladle the soup into oven-safe bowls, leaving about an inch from the top. Place a toasted bread slice on top of each bowl. Sprinkle the shredded Gruyère cheese generously over the bread. Place the bowls under the broiler for 3-5 minutes, or until the cheese is melted and bubbly.
Serve: Carefully remove the bowls from the oven and let them cool slightly before serving.
Notes
Ensure all ingredients, especially the broth and cheese, are certified kosher to maintain the integrity of the dish.