• Chop all the onions and set aside.
• In a large Dutch oven or pot, melt the butter over medium heat. Add the oil, onions, salt, pepper. Cook, on medium heat, uncovered, stirring occasionally with a wooden spoon, until onions are deep golden brown and caramelized, approximately 70 minutes.
  
• Add the vermouth or wine and deglace the pan. Cook, stirring with a wooden spoon to scrape any food from the bottom of the pan, until almost all of the liquid has evaporated approximately 10 minutes.
  
• Add the broth, Worcestershire sauce, thyme, and bay leaves to the pot. Bring to a boil, reduce the heat to a simmer, and cook, covered, for about 1.5 hours.

Prepare the Bread Slices

• While the soup simmers, preheat the oven to 350°F and set an oven rack in the middle position. Arrange the baguette slices in a single layer on a baking sheet and bake until the bread is dry, crisp, and golden at edges, about 12 minutes. Set aside.

Finish the Soup

• When the soup is finished cooking, remove the bay leaves and taste and adjust seasoning if necessary. If the soup needs a deeper flavor, try a few shakes of Worcestershire sauce.
• Preheat the oven to 375 degrees.
• Place oven proof bowls on a baking sheet. Place a baguette slice into each oven-proof bowls add a little parmesan cheese, then fill them with the very hot soup, top each bowl with 1 or 2 baguette slices without overlapping, and sprinkle evenly with Gruyère and Parmigiano Reggiano cheese.
• Bake for 30 minutes or until cheese is beautifully browned. In the last few moments, place the remaining baguette slices in the oven to warm.

  Serve the soup immediately with the additional toasted slices of baguette.