French Onion Green Bean Casserole

Name: French Onion Green Bean Casserole
Ingredients: Two 10.8-ounce bags frozen whole green beans; 4 tablespoons unsalted butter; 2 medium onions, thinly sliced; 1 tablespoon sherry vinegar; 1 tablespoon fresh thyme leaves, chopped; One 1-ounce packet French onion soup mix; 1 tablespoon all-purpose flour; Kosher salt and freshly ground black pepper; 1 1/2 cups low-sodium beef broth; 1/2 cup heavy cream; One 6-ounce container crispy fried onions, such as French’s; 8 ounces Gruyère, shredded (about 2 cups); 1/2 cup grated Parmesan