• Melt butter and 2 Tablespoons extra virgin olive oil in a large, 6-quart Dutch oven or heavy-bottomed pot. Add onions and a generous pinch of salt then stir to coat and place a lid on top. Cook for 20 minutes, stirring halfway through, then remove lid and continue sautéing onions for 1 - 1-1/2 hours, stirring every 10-15 minutes, or until onions are very tender and caramelized. Turn the heat down if necessary to avoid searing the onions.
• Turn heat up to medium-high then sprinkle in flour and cook for 1 minute, stirring constantly. Add wine and worcestershire sauce then simmer until mixture has thickened and the liquid has nearly evaporated, 1-2 minutes. Add beef broth, chicken broth, thyme, and bay leaf then turn the heat up to high to bring the soup to a boil. Turn heat down to medium/medium-low then gently simmer for 1 hour, occasionally stirring and skimming off fat from the top if necessary. Remove bay leaf and thyme stems then season with salt and pepper to taste.

For the Bread Cubes:

• Preheat oven to 325 degrees then line a half sheet pan with parchment paper or a silpat. Cut bread slices into cubes then add to the sheet pan and drizzle with extra virgin olive oil and salt. Toss with your fingers to coat then spread cubes out and bake for 14-15 minutes, stirring once, or until golden brown and crunchy.
• Switch oven to broiler mode then place an oven rack in the second highest spot. Scoop soup into 4, oven-proof crocks then top with croutons, a slice of swiss cheese, and a quarter of the grated gruyere cheese, letting some hang off the edge of the crocks. Place crocks back onto prepared sheet pan then broil until cheese is golden brown and bubbly. Serve immediately.