Begin by slicing the onions thinly, ensuring they are uniform for even caramelization. Next, cut the beef short ribs into manageable sections, about 3-4 inches long. Gather your ingredients, including fresh thyme and bay leaves.

In a large, heavy-bottomed pot, melt 2 tablespoons of butter over medium heat. Add the sliced onions and cook for about 30 minutes, stirring occasionally, until they transform into a golden brown color.

Once the onions are caramelized, push them to one side of the pot and add the beef short ribs. Sear the ribs for 5-7 minutes, turning until browned on all sides.

Pour in the beef broth, scrape up browned bits, and add fresh thyme and bay leaves. Bring to a gentle boil, then reduce heat to low and simmer uncovered for about 2 hours.

Preheat your oven to 350°F (175°C). After 2 hours, ladle the soup into oven-safe bowls, topping with slices of sourdough followed by Gruyère cheese. Bake for 15-20 minutes until cheese is bubbly and golden.