French Onion Soup Casserole

Name: French Onion Soup Casserole
Prep time: 10 min
Cook time: 1 h 10 min
Total time: 1 h 20 min
Cuisine: European, Mediterranean, French
Ingredients: 1/4 cup unsalted butter; 3 lbs Vidalia onions, thinly sliced (about 5 medium); 2 teaspoons Kosher salt; 1/2 teaspoon black pepper; 3 sprigs thyme, plus more for topping; 2 bay leaves; 1 baguette, thinly sliced; 1/3 cup all-purpose flour; 3 cups beef broth; 1/2 cup sherry; 2 cups Gruyere cheese, grated