In a large skillet, heat 2 tablespoons of butter and 1 tablespoon of olive oil over medium heat. Add 3 large sliced onions, along with 2 sprigs of fresh thyme and 1 bay leaf. Sauté gently on low heat for about 45 minutes, stirring every 10 minutes, until the onions are a deep golden brown and have caramelized beautifully. Sprinkle with salt for flavor enhancement during the process.

Once your onions are perfectly caramelized, pour in 1/2 cup of white wine and increase the heat to high to bring it to a gentle boil. Allow the alcohol to cook off for about 2-3 minutes, scraping the browned bits from the bottom of the skillet for added flavor. Then, add 6 cups of beef broth and let the mixture simmer on low for 20-30 minutes.

Preheat your broiler to high. Grab 4 oven-safe bowls and ladle the hot soup into each one. Top each bowl with a slice of toasted baguette, then generously sprinkle with grated Gruyère cheese.

Carefully place the bowls under the broiler for 3-5 minutes, watching closely as the cheese melts, bubbles, and turns golden brown.

After broiling, remove the bowls from the oven and, if desired, add a few fresh thyme leaves on top for an aromatic touch. Serve immediately.