Cheesy French Onion Meatballs

Name: Cheesy French Onion Meatballs
Ingredients: 2 pounds yellow onions (about 2 large or 4 medium); 1 small bunch fresh thyme; 2 cloves garlic; 6 ounces Gruyère cheese, divided; 1/2 cup panko breadcrumbs; 1/2 cup whole or 2% milk; 1 large egg; 2 teaspoons Worcestershire sauce; 2 1/2 teaspoons kosher salt, divided, plus more as needed; 3/4 teaspoon freshly ground black pepper, divided, plus more as needed; 1 1/2 pounds ground beef; 2 tablespoons unsalted butter; 2 tablespoons all-purpose flour; 1/2 cup dry white wine; 1 (14.5-ounce) can beef broth (scant 2 cups); 1 teaspoon balsamic or sherry vinegar; Crusty bread, for serving (optional)