French Onion Meatballs - One-Pan Comfort Food Recipe

Ingredients

• 2 pounds yellow onions (about 2 large or 4 medium).
• 1 small bunch fresh thyme.
• 2 cloves garlic, minced.
• 6 ounces Gruyère cheese, grated.
• 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.
• 3/4 teaspoon black pepper.
• 1 1/2 pounds ground beef.
• 2 tablespoons unsalted butter.
• 2 tablespoons all-purpose flour.
• 1/2 cup dry white wine.
• 14 1/2 ounces beef broth.
• 1 teaspoon balsamic or sherry vinegar.