In a large stockpot, heat the butter and olive oil, then add the onions and cook over medium heat. Stir them around about every 10 minutes. After 25-30 minutes, they will start to develop some color and caramelize. Keep cooking them until they are a dark amber color and are sweet. Season the onions with salt and pepper. Add the Worcestershire sauce, beef broth, bay leaf, and thyme. Bring to a boil, then simmer for 45 minutes.
While the soup simmers, slice the baguette and rub with olive oil. Sprinkle with salt and pepper and toast under the broiler.
When the soup is ready, ladle it into individual oven proof dishes, top with 1-2 sliced of toasted baguette, and about 1/4 C of shredded gruyere. Place under broiler until cheese is melty and starting to brown. Serve immediately.