Ingredients List:

 4 medium yellow onions, thinly sliced (about 6 cups)

 3 cloves garlic, thinly sliced

 2 bay leaves

1 1/2 teaspoons kosher salt
 1 teaspoon fresh thyme leaves
 1/2 teaspoon freshly ground black pepper
 1/3 cup dry white wine, such as sauvignon blanc
 2 tablespoons unsalted butter
 1 pound 90% lean ground beef
 1 large egg, beaten
 1/3 cup panko breadcrumbs
 1 tablespoon all-purpose flour
 2 cups low-sodium chicken broth
 1 teaspoon Dijon mustard
 1 teaspoon Worcestershire sauce
 6 ounces freshly grated Gruyere cheese (about 1 cup)
 Toasted baguette slices, for serving
Instructions:
 Cook the onions: In a skillet, heat olive oil, add onions, garlic, bay leaves, and salt. Cook until golden, about 45 minutes. Add thyme, black pepper, white wine, and butter. Remove bay leaves, let cool.
 Make the meatball mixture: Combine ground beef, egg, panko, salt, pepper, and half of the cooked onions. Mix well.
 Form meatballs: Divide mixture into 16 balls.
 Cook meatballs: In the skillet, brown meatballs on all sides, about 10-12 minutes. Sprinkle flour, add chicken broth, remaining onions, mustard, and Worcestershire. Simmer until sauce thickens.
 Add cheese and broil: Sprinkle Gruyere over meatballs and broil until melted.
 Tips:
 For a twist, try different cheeses like Emmental or Swiss for varied flavors.
 Serve with garlic-rubbed toasted baguette slices for an added crunch.