Mix & Shape Meatballs: In a bowl, soak panko breadcrumbs in milk for 5 minutes. Add ground beef, egg, grated parmesan, minced garlic, chopped thyme, Worcestershire, kosher salt, and black pepper. Gently combine with hands without overworking. Roll into 20 golf ball-sized meatballs (approx. 1.5 inches).

Sear the Meatballs: Heat olive oil in a large 12-inch oven-safe cast iron skillet over medium-high heat. Add meatballs in batches and sear for 5 to 6 minutes, turning until browned on all sides (they don't need to cook through yet). Transfer to a plate.

Caramelize the Onions: Lower heat to medium-low. Melt butter in the same skillet. Add sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for 20 to 25 minutes until deep golden-brown, soft, and caramelized.

Build the French Onion Gravy: Sprinkle flour over the caramelized onions and stir for 1 minute to cook out raw flour taste. Slowly pour in beef bone broth, whisking constantly to dissolve browned bits on the pan bottom. Whisk in Dijon mustard and add fresh thyme sprigs. Bring to a gentle simmer until gravy begins to thicken.

Simmer Meatballs: Nestled the seared meatballs back into the bubbling onion gravy. Cover loosely with foil and simmer over low heat for 10 minutes until meatballs are thoroughly cooked through (165°F).

Melt & Broil Cheese: Remove thyme sprigs. Generously smother the top of the meatballs and gravy with the shredded Gruyère and Swiss cheeses. Transfer the skillet directly under your oven broiler on HIGH for 3 to 4 minutes until the cheese is bubbling, gooey, and beautifully blistered golden-brown. Garnish with fresh thyme and serve hot with crusty baguette!