French Onion Soup With Braised Short Ribs

Name: French Onion Soup With Braised Short Ribs
Ingredients: 2 pounds bone-in beef short ribs; 2 tablespoons all-purpose flour; Kosher salt and freshly ground pepper; 2 tablespoons canola oil; 1 carrot, chopped; 1 medium yellow onion, chopped; 1 leek, chopped; 4 sprigs thyme, plus 1 tablespoon thyme leaves; 1 bay leaf; 1 head garlic, halved crosswise; 1 tablespoon tomato paste; 1 cup dry red wine; 6 cups low-sodium beef stock; 1 stick unsalted butter; 6 Spanish onions, thinly sliced; 3/4 cup dry sherry; 1 tablespoon sherry vinegar; 12 slices sourdough bread, toasted; 12 slices high-quality gruyere cheese (about 6 ounces); Chopped fresh chives, for topping