Heat a Dutch oven over medium-high heat and add olive oil. Once shimmering, season the beef short ribs generously with salt and pepper, then sear them for 3-4 minutes on each side until deeply browned. Remove the ribs and set aside.

In the same pot, reduce the heat to medium and add butter, allowing it to melt. Add thinly sliced onions and stir gently, cooking for 25-30 minutes until soft and golden brown.

Add minced garlic to the pot and sauté for an additional 1-2 minutes until fragrant. Deglaze the pot by adding a splash of beef broth.

Return the seared beef short ribs to the Dutch oven along with remaining beef broth, Worcestershire sauce, bay leaf, and thyme. Stir well and bring to a gentle simmer.

Cover the pot and let the soup simmer on low heat for about 3 hours, stirring occasionally.

Remove the ribs, discard the bay leaf, and shred the meat from the bones. Return the tender beef to the soup.

Spoon the soup into oven-safe bowls, top each with a slice of toasted baguette and sprinkle with Gruyère cheese.

Place bowls under broiler for 2-3 minutes until the cheese is melted and bubbly.

Remove from oven and let cool slightly. Serve hot garnished with fresh thyme if desired.