In a large pot or Dutch oven, heat the olive oil over medium-high heat. Season the short ribs with salt and pepper, then add them to the pot. Sear the ribs on all sides until browned, about 8-10 minutes. Remove the ribs and set aside.
In the same pot, add the sliced onions. Cook over medium heat, stirring frequently, until caramelized and golden brown, about 20-25 minutes. Add the minced garlic and cook for an additional 1-2 minutes.
Return the short ribs to the pot. Pour in the beef broth, red wine, and Worcestershire sauce. Add the thyme and bay leaf. Bring the mixture to a simmer, then cover and reduce the heat to low. Let it cook for 2-3 hours, or until the short ribs are tender and easily pull apart.
Once the short ribs are done, remove them from the pot and shred the meat, discarding the bones. Return the shredded meat to the soup and stir to combine. Adjust seasoning with additional salt and pepper if needed.
Preheat the broiler. Ladle the soup into oven-safe bowls, top each with a slice of 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 brown.
Remove from the oven and let cool slightly. Garnish with chopped parsley before serving.
Notes
For a richer flavor, consider adding a tablespoon of tomato paste when caramelizing the onions.
You can substitute the Gruyère cheese with Swiss cheese or a blend of mozzarella and Parmesan for a different flavor profile.