To cook the onions:

1. In a large oven-safe skillet, heat the olive oil over medium heat and add the onions, garlic, bay leaves, and salt and cook, stirring frequently, until the onions are deeply golden (about 45 minutes)
2. Add the thyme and black pepper and cook until the herbs are fragrant, about 1 minute
3. Add the white wine and butter, scraping up the bottom of the pan to loosen any dark bits
4. Remove onions from skillet and discard the bay leaves and allow the onions to cool slightly
5. Rinse and wipe out the skillet and set aside

Make the meatballs:

1. In a large bowl, combine the ground beef, egg, flour or breadcrumbs, salt, and pepper
2. Once the onions are cool enough to handle, roughly chop 1/3 of the onions and add them to the meatball mixture
3. Use your clean hands or meat handling gloves to mix everything together until combined
4. Divide the mixture up into 12 roughly 1 1/2-inch balls and set aside
5. Heat 2 tablespoons of olive oil in the reserved skillet over medium-high heat
6. Add the meatballs, making sure to space them evenly, and cook until browned on all sides, 10 to 12 minutes

To combine the dish and finish up:

1. Sprinkle flour over the meatballs and toss gently until the flour is cooked, about 1 minutes
2. Add the chicken broth, remaining reserved onions, mustard, and Worcestershire
3. Stir well, and let simmer until the sauce has thickened and grown slightly darker in color, about 10 minutes
4. Preheat the broiler on low
5. Sprinkle the Gruyere over the meatballs and sauce and broil until the cheese is melted, about 2 minutes
6. Serve with toasted baguette slices or as is. Garnish with more thyme if desired