In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add thinly sliced onions, minced garlic, bay leaves, and a pinch of salt. Sauté for about 45 minutes, stirring occasionally, until the onions are golden brown and caramelized.

Stir in fresh thyme and black pepper. Pour in half a cup of dry white wine and a tablespoon of butter, scraping the brown bits off the bottom. Allow to cook for another 2-3 minutes until the wine is mostly evaporated.

Transfer the onion mixture to a bowl and let it cool for a few minutes. Combine 1 pound of ground beef, a beaten egg, panko breadcrumbs, and additional salt and pepper. Fold in the caramelized onions.

Preheat the skillet over medium heat, adding a drizzle of olive oil. Shape the mixture into 16 equal meatballs. Place them in the skillet, browning for about 10-12 minutes.

Sprinkle 1 tablespoon of flour over the browned meatballs and toss to coat. Pour in 1 cup of low-sodium chicken broth, stirring gently. Add the remaining caramelized onion mixture, Dijon mustard, and Worcestershire sauce. Simmer for about 10 minutes.

Preheat your broiler to high. Transfer the meatballs into a baking dish, or keep them in the skillet if broiler-safe. Sprinkle grated Gruyere cheese over the top. Broil for 2-3 minutes until bubbly and golden.

Remove from the oven and garnish with fresh parsley. Serve hot.