French Onion Soup

Name: French Onion Soup
Cook time: 1 h 50 min
Total time: 1 h 50 min
Cuisine: French
Ingredients: 3 tbsp. olive oil; 4 pounds yellow onions (about 5 large onions), thinly sliced; Kosher salt; 1 1/2 tsp. all-purpose flour; 1/4 c. cognac or dry white wine; 2 32-oz containers low-sodium beef broth; 2 bay leaves; 6 large sprigs thyme, plus more for sprinkling; 1 tbsp. sherry vinegar; 8 thin slices country bread; 3 oz. Gruyère cheese, finely grated