In a large skillet, heat the olive oil over medium heat. Add the onions, garlic, bay leaves, and salt. Cook, stirring frequently, until the onions are deeply golden, about 45 minutes. Add a splash of water if it becomes dry.

Stir in the thyme and black pepper, cooking until fragrant, about 1 minute. Pour in the white wine and add the butter, scraping the pan's bottom. Remove and discard the bay leaves, then transfer the onions to a plate to cool slightly. Rinse and wipe out the skillet.

In a large bowl, combine the ground beef, egg, panko, salt, and pepper. Once the onions have cooled, roughly chop half and mix them into the meatball mixture until combined.

Divide the meat mixture into 16 roughly 1 1/2-inch balls, ensuring they’re uniform for even cooking.

Heat 2 tablespoons of olive oil in the reserved skillet over medium-high heat. Add the meatballs and cook until browned on all sides, about 10 to 12 minutes.

Sprinkle flour over the meatballs, tossing gently until the flour is cooked, about 1 minute. Add the chicken broth, remaining onions, mustard, and Worcestershire sauce. Stir well and let simmer until the sauce thickens, around 10 minutes.

Preheat the broiler on low. Sprinkle the Gruyere cheese over the meatballs and sauce, broiling until the cheese is melted, about 2 minutes.

Enjoy your French Onion Meatballs with toasted baguette slices for the perfect dipping experience.