Preheat your oven to 325°F (160°C) to prepare for the braising process.

Season the short ribs with kosher salt and freshly ground pepper. Heat olive oil in a heavy, oven-safe pot over medium-high heat and brown the short ribs for 8-10 minutes.

In the same pot, add chopped carrots and sliced onions or leeks, sautéing for 5-7 minutes until caramelized.

Stir in tomato paste and cook for 1 minute, then add red wine and simmer for 3-5 minutes.

Return short ribs to the pot, add garlic, thyme, and bay leaf. Pour in enough beef broth to cover the meat and braise in the oven for 2.5 to 3 hours.

Meanwhile, melt butter in a skillet and cook sliced onions over low heat for 40-90 minutes until deep brown.

Once ribs are tender, shred the meat and strain the broth, returning it to the pot with caramelized onions.

Simmer the combined soup for about 10 minutes, adjusting seasoning as needed.

Preheat your broiler, slice the bread, top with gruyère cheese, and broil for 2-4 minutes.

Ladle soup into bowls, top with cheese toast and garnish with chives.