Step 1: Pat the short ribs dry with paper towels. Season generously with 1/2 tsp salt and 1/4 tsp black pepper.
Step 2: Heat 2 tbsp olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Sear the short ribs on all sides until deeply browned, about 3-4 minutes per side. Remove ribs and set aside.
Step 3: Reduce heat to medium-low. Add 4 tbsp butter to the pot. Add the sliced onions, remaining 1/2 tsp salt, 1/4 tsp pepper, and granulated sugar. Cook, stirring occasionally, for 30-45 minutes, or until the onions are deeply caramelized and a rich golden brown. Be patient, as this step is crucial for flavor.
Step 4: Pour in the brandy or sherry, scraping up any browned bits from the bottom of the pot. Cook for 2-3 minutes until mostly evaporated.
Step 5: Add the dry red wine and simmer for another 5 minutes, allowing it to reduce slightly.
Step 6: Return the seared short ribs to the pot. Pour in the beef broth, add bay leaves, fresh thyme, and Dijon mustard. Bring to a gentle simmer.
Step 7: Cover the Dutch oven and transfer to a preheated oven at 325°F (160°C). Braise for 2.5 to 3 hours, or until the short ribs are fall-off-the-bone tender. Alternatively, simmer gently on the stovetop over low heat, covered, for the same duration.
Step 8: Carefully remove the short ribs from the pot. Discard the bay leaves and thyme sprig. Skim any excess fat from the surface of the soup, if desired.
Step 9: Shred the short rib meat from the bones and discard bones. Return the shredded meat to the soup.
Step 10: Prepare the Gruyère toast: Preheat your oven's broiler. Arrange baguette slices on a baking sheet. Brush lightly with 2 tbsp olive oil and toast in the broiler for 1-2 minutes per side until lightly golden. Remove from oven.
Step 11: Top each toasted baguette slice generously with shredded or sliced Gruyère cheese. Return to the broiler for 1-3 minutes, or until the cheese is bubbly and golden brown.
Step 12: Ladle the hot French Onion Short Rib Soup into bowls. Top each serving with one or two slices of the cheesy Gruyère toast and serve immediately. Enjoy your ultimate comfort bowl!