Begin by heating a large Dutch oven over medium-high heat. Add a splash of oil and carefully place the short ribs in the pot, searing them for about 4-5 minutes on each side until they develop a rich, brown crust.

Once the short ribs are browned, remove them from the pot and set aside. In the same pot, add thinly sliced onions and a pinch of salt, stirring occasionally. Cook for 15-20 minutes on medium heat until they turn golden brown and caramelized.

After caramelizing the onions, pour in a splash of beef broth to deglaze the pot, scraping up any tasty browned bits stuck to the bottom.

Return the seared short ribs to the pot, along with the remaining beef broth and fresh thyme. Bring the mixture to a gentle simmer, then cover and reduce the heat to low. Allow the soup to cook for about 1.5 to 2 hours.

While the soup simmers, preheat your oven to 375°F (190°C). Slice the baguette into thick pieces, then spread a thin layer of butter on one side of each slice. Place the slices on a baking sheet, butter side up, and top generously with shredded Gruyère cheese. Bake for 10-12 minutes.

Once the short ribs are tender, remove them from the pot and shred the meat, discarding the bones. Return the meat to the soup and stir to combine. Serve the soup in bowls, topping each with a slice of Gruyère toast.