1. Preheat your oven to 325°F (165°C).
2. In a Dutch oven, heat the olive oil and butter over medium heat. Add the beef short ribs and brown on all sides. Remove and set aside.
3. In the same pot, add the sliced onions and cook until caramelized, about 25 minutes. Add the garlic and cook for another 2 minutes.
4. Pour in the red wine to deglaze the pot, scraping up any browned bits. Let it simmer for 5 minutes.
5. Add the beef broth, thyme, salt, and pepper. Return the short ribs to the pot. Bring to a simmer, cover, and transfer to the oven. Cook for 2.5 hours.
6. Remove the pot from the oven. Take out the short ribs, shred the meat, and discard the bones. Return the meat to the pot.
7. Ladle the soup into oven-safe bowls. Top with a slice of French bread and a generous amount of grated Gruyère cheese.
8. Place the bowls under the broiler until the cheese is melted and bubbly. Serve hot.