Step 1: Prepare the Meatball Mixture

In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, chopped onion, egg, minced garlic, dried thyme, salt, and pepper. Mix everything together gently using your hands or a spoon, being careful not to overwork the mixture. The goal is to combine the ingredients just enough to hold the meatballs together without making them too dense.

Once the mixture is well combined, add beef broth to the mix, which will help keep the meatballs moist. Stir until the mixture is cohesive but still tender. You want the meatballs to be soft and juicy, not tough.

Step 2: Form and Cook the Meatballs

Use your hands or a small cookie scoop to form the meat mixture into meatballs about 1 1/2 inches in diameter. You should get about 24 meatballs, depending on the size. Place the meatballs on a plate or tray while you heat the oil.

In a large skillet, heat olive oil over medium-high heat. Once the oil is hot, carefully place the meatballs in the skillet, making sure not to overcrowd the pan. Cook in batches if necessary. Sear the meatballs on all sides, about 5-6 minutes, until they’re golden brown and cooked through. You don’t need to cook them all the way through at this point, as they will finish cooking in the sauce. Once cooked, transfer the meatballs to a paper towel-lined plate to drain any excess oil.

Step 3: Caramelize the Onions

For the French onion sauce, start by heating olive oil and butter in a large skillet over medium heat. Once the butter has melted, add the sliced onions. Stir them occasionally to coat them in the butter and oil. Sprinkle in the sugar, which will help caramelize the onions and give them a rich, sweet flavor.

Continue to cook the onions for 25-30 minutes, stirring occasionally. They should slowly turn golden brown and become very tender. Be patient during this step, as caramelizing onions takes time, but the results are well worth it. The onions will shrink as they cook and develop a deep, rich flavor.

Step 4: Build the French Onion Sauce

Once the onions are caramelized, add the minced garlic and cook for another 1-2 minutes, until fragrant. Stir in the beef broth, white wine, and Worcestershire sauce, scraping up any browned bits from the bottom of the skillet. Add the dried thyme, salt, and pepper, and bring the sauce to a simmer.

Allow the sauce to cook for about 10 minutes to reduce slightly and thicken. The flavors will meld together beautifully, and the sauce will become rich and savory, much like the broth in French onion soup.

Step 5: Combine the Meatballs and Sauce

Return the browned meatballs to the skillet with the onion sauce, ensuring they are submerged in the sauce. Spoon some of the sauce over the meatballs and let them simmer for another 10-15 minutes, or until the meatballs are cooked through and the flavors have melded together.

Step 6: Add Cheese and Broil

Once the meatballs are cooked through, sprinkle the Gruyère cheese evenly over the top. You can also sprinkle a little extra Parmesan cheese if you like a more intense cheesy flavor. Carefully transfer the skillet to the oven (or use a separate baking dish if your skillet is not oven-safe) and broil for 2-3 minutes, or until the cheese is melted and bubbly, creating a golden, crispy layer on top of the meatballs.

Step 7: Serve and Garnish

Once the cheese is melted and the meatballs are sizzling, remove from the oven and let them rest for a couple of minutes. Garnish with fresh parsley to add a pop of color and a fresh flavor contrast.

Serve the meatballs immediately, and enjoy them with crusty bread for dipping, over mashed potatoes, or as part of a larger spread of appetizers.