Sauté the Onions: In a large pot or Dutch oven, combine the butter and olive oil over medium heat. Once melted, add the sliced onions, kosher salt, and black pepper. Cook uncovered for 45–55 minutes, stirring every few minutes until caramelized.

Deglaze the Pot: After the onions are caramelized, pour in the dry white wine and sherry. Increase heat to medium-high and bring to a boil. Cook for 5–6 minutes, scraping the browned bits from the bottom.

Incorporate the Broth: Sprinkle the flour over the onion mixture and stir for about 1 minute. Gradually add 1 cup of beef broth, stirring constantly, then add the remaining broth, bay leaf, thyme, and Worcestershire sauce. Stir until well integrated.

Simmer the Soup: Bring the soup to a boil, then reduce the heat to low, cover, and simmer for 20 minutes. Taste and adjust seasoning with salt or pepper if needed.

Prepare the Baguette: Preheat your oven to 400°F (200°C). Arrange sliced baguette on a baking sheet and bake for 8–10 minutes until crispy and golden.

Final Touches: Remove the bay leaf and thyme sprigs. Portion the soup into ovenproof bowls, add baguette slices, and sprinkle with grated cheese.

Broil to Perfection: Set your oven to broil and place the bowls under the broiler. Broil for 2–5 minutes until cheese is melted and bubbly.