In a heavy Dutch oven, heat a tablespoon of olive oil over medium-high heat until shimmering. Add the beef short ribs, browning them on all sides for about 8-10 minutes until they develop a rich, golden crust.

Reduce the heat to medium, and add two tablespoons of butter to the drippings left in the pot. Stir in the sliced yellow onions, cooking them slowly for 30-40 minutes until they become soft and deeply golden brown.

Once the onions are beautifully caramelized, add minced garlic and sauté for about 1 minute until fragrant. Pour in a cup of dry red wine, scraping the bottom of the pot to release any browned bits.

Return the seared short ribs to the pot, and pour in approximately 6 cups of low-sodium beef broth. Add diced carrots, celery, fresh thyme, and a bay leaf. Increase the heat to bring it to a gentle boil.

Once boiling, reduce to low, covering the pot, and let it simmer for 2.5 to 3 hours until the ribs are fork-tender.

Carefully lift the short ribs from the pot and allow them to cool slightly. Shred the meat off the bones, discarding the bones and any excess fat.

Preheat your broiler to high. Slice a fresh baguette into thick slices and arrange them on a baking sheet. Generously top each slice with grated Gruyère cheese. Broil for 2-4 minutes, keeping a close eye until the cheese is bubbly and golden.

Ladle the soup into warm bowls, ensuring each serving includes tender meat and sweet caramelized onions. Add a cheesy Gruyère toast on top of each bowl.