French Onion Meatballs with Gruyere (Cozy Skillet Dinner) - Quickie Recipe

Ingredients

• 1/2 cup (35 g) panko breadcrumbs
• 1/3 cup (80 ml) whole milk
• 1 large egg
• 1/4 cup (25 g) finely grated Parmesan cheese
• 1/4 cup (40 g) grated yellow onion
• 2 cloves garlic, minced
• 1 teaspoon Dijon mustard
• 1 teaspoon Worcestershire sauce
• 1 teaspoon fresh thyme leaves, or 1/2 teaspoon dried thyme
• 1 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 1 1/2 lb (680 g) ground beef, 85 percent lean
• 2 tablespoons unsalted butter
• 1 tablespoon olive oil
• 3 large yellow onions (about 1 1/2 lb or 680 g), thinly sliced
• 1/2 teaspoon kosher salt, plus more to taste
• 2 tablespoons all-purpose flour
• 1/2 cup (120 ml) dry white wine, or more beef broth
• 2 cups (480 ml) beef broth
• 1 teaspoon balsamic vinegar or sherry vinegar
• 1 1/2 cups (170 g) shredded Gruyere cheese
• 1 tablespoon chopped fresh parsley, for serving