Step 1: Prepare the Meatballs

1. Preheat the Oven:

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper or lightly grease it.

2. Mix the Meatball Ingredients:

In a large mixing bowl, combine the ground beef, breadcrumbs, egg, grated Parmesan, minced garlic, Worcestershire sauce, Dijon mustard, thyme, onion powder, and salt and pepper. Mix everything together gently using your hands or a large spoon, making sure not to overmix, as this can make the meatballs tough.

3. Shape the Meatballs:

Using your hands, shape the meat mixture into 1 ½-inch meatballs, about the size of a golf ball. You should get about 18-20 meatballs from this mixture. Place the meatballs on the prepared baking sheet, making sure they’re spaced evenly apart.

4. Bake the Meatballs:

Bake the meatballs in the preheated oven for 15-18 minutes, or until they’re browned on the outside and cooked through. You don’t have to worry about them being completely done at this point since they’ll continue cooking in the sauce later.

Step 2: Caramelize the Onions

1. Slice the Onions:

While the meatballs are baking, slice the onions thinly. You want the slices to be uniform in thickness so they cook evenly.

2. Caramelize the Onions:

In a large skillet, melt the butter over medium heat. Add the sliced onions and stir to coat them in the butter. Cook the onions over medium heat, stirring occasionally, for about 20-25 minutes, or until they are soft, golden brown, and caramelized.

3. Add Garlic and Seasonings:

Once the onions are caramelized, stir in the minced garlic and cook for another 1-2 minutes until fragrant. Sprinkle the onions with the flour and stir to coat the onions evenly. This will help thicken the sauce later.

Step 3: Make the French Onion Sauce

1. Add the Broth:

Slowly pour in the beef broth, stirring constantly to avoid lumps from the flour. Add the Worcestershire sauce, thyme, salt, and pepper. Stir everything together and bring the mixture to a simmer.

2. Simmer the Sauce:

Let the sauce simmer for 5-7 minutes until it thickens slightly. Taste and adjust the seasoning as needed. If the sauce becomes too thick, you can add a splash of water or more broth to reach your desired consistency.

Step 4: Combine the Meatballs and Sauce

1. Add the Meatballs to the Sauce:

Once the meatballs are done baking, carefully transfer them into the skillet with the French onion sauce. Spoon some of the sauce over the meatballs to coat them evenly.

2. Top with Cheese:

Sprinkle the shredded Gruyère or mozzarella cheese over the meatballs and sauce. Place the skillet under the broiler for 2-3 minutes, or until the cheese is melted and bubbly.

Step 5: Garnish and Serve

Once the cheese is melted and golden, remove the skillet from the oven. Garnish with chopped parsley and serve the meatballs hot with crusty bread, mashed potatoes, or over a bed of buttery noodles.