In a large Dutch oven, heat 2 tablespoons of olive oil over medium-high heat. Add the short ribs and sear for 3-4 minutes on each side until golden-brown. Remove and set aside.

In the same pot, add sliced onions and stir to coat in the fond. Lower heat to medium and cook for 15-20 minutes until they caramelize.

Return the seared short ribs to the pot. Add 4 cups of beef broth, 2 teaspoons of thyme, and 2 bay leaves. Stir gently and scrape up any bits stuck to the bottom.

Bring to a boil, then reduce heat to low, cover, and let simmer for about 3 hours until the ribs are tender.

Ladle into bowls, top with shredded Gruyère cheese, and broil for 2-3 minutes until cheese is bubbly and golden.