French Onion Meatballs - Easy One-Pan Dinner

Ingredients

• 2 big yellow onions (about 2 whole large or 4 smaller ones).
• A small handful of fresh thyme sprigs.
• 2 minced garlic cloves.
• 6 oz Gruyère cheese, shredded.
• 1/2 cup of panko crumbs.
• Half a cup of 2% or whole milk.
• An egg, large size.
• 2 teaspoons of Worcestershire sauce.
• 2 1/2 teaspoons kosher salt.
• 3/4 teaspoon ground black pepper.
• 1 1/2 lbs ground beef.
• 2 tablespoons butter, unsalted.
• 2 tablespoons plain flour.
• 1/2 cup dry white wine.
• 14 1/2 oz beef stock.
• 1 teaspoon vinegar (sherry or balsamic works).