Preheat your oven to 350°F. In a large oven-safe pot, heat the olive oil over medium-high heat. Season the short ribs with salt and pepper, then sear them in the pot until browned on all sides, about 3-4 minutes per side. Remove the ribs and set aside.
In the same pot, add the sliced onions and cook, stirring frequently, until they are caramelized and golden brown, about 20-25 minutes. Add the minced garlic, thyme, and rosemary, and cook for an additional 2 minutes.
Pour in the white wine, scraping the bottom of the pot to deglaze it. Let it simmer for about 5 minutes until the wine reduces slightly.
Return the short ribs to the pot, then add the beef broth and Worcestershire sauce. Bring the mixture to a simmer, cover, and transfer the pot to the preheated oven. Cook for 2-2.5 hours, or until the meat is tender and falling off the bone.
Once the soup is ready, remove the pot from the oven. Carefully take out the short ribs, shred the meat, and discard the bones. Return the shredded meat to the pot and stir to combine.
Preheat the broiler. Place the baguette slices on a baking sheet and broil until golden brown, about 1-2 minutes per side.
To serve, ladle the soup into bowls, top each bowl with a slice of toasted baguette, and sprinkle generously with Gruyère cheese. Place the bowls under the broiler for 2-3 minutes, or until the cheese is bubbly and golden.