Prepare the Bread

• Preheat your oven to 375°F (190°C).
• Slice the baguette into ½-inch thick pieces.
• Toast in the oven for 10 minutes until golden brown and crisp. Set aside.

Caramelize the Onions

• Melt butter and olive oil in a large pot or Dutch oven over medium heat.
• Add onions, salt, and black pepper and cook, stirring occasionally, for 30-40 minutes until the onions turn deep golden brown and caramelized.

Add Flour and Deglaze

• Sprinkle flour over the onions and stir well.
• Cook for 2-3 minutes to remove the raw flour taste.
• Pour in the white wine, scraping up any browned bits from the bottom of the pot.
• Simmer for 5 minutes until the wine reduces slightly.

Simmer the Soup

• Add beef stock, thyme sprigs, and cognac (if using).
• Bring to a rolling boil, then reduce heat and let it simmer for 30 minutes.
• Taste and adjust seasoning with more salt and pepper if needed.

Assemble and Broil

• Preheat your broiler.
• Ladle the soup into oven-proof bowls.
• Top each bowl with a toasted baguette slice.
• Sprinkle generously with Gruyère and Parmesan cheese.

Broil the Cheese

• Place the bowls under the broiler for 3-5 minutes, or until the cheese is golden, bubbly, and slightly crispy.
• Watch closely to prevent burning.

Garnish and Serve

• Remove from the broiler and garnish with chopped chives.
• Serve immediately and enjoy the deep, rich flavors of this classic soup!