Mix & Shape Meatballs: In a large bowl, soak panko breadcrumbs in milk for 3 minutes until softened (this panade keeps meatballs ultra-tender). Add ground beef, ground pork, beaten egg, minced garlic, grated parmesan, Worcestershire sauce, salt, and pepper. Gently mix with clean hands just until combined (do not overwork). Roll into 15-18 golf ball-sized meatballs.

Sear the Meatballs: Heat olive oil and 1 tablespoon butter in a 12-inch oven-safe cast iron skillet over medium-high heat. Add meatballs in a single layer and sear for 5-6 minutes, turning occasionally until golden brown on all sides (they do not need to be cooked through completely yet). Transfer to a plate.

Caramelize the Onions: Lower skillet heat to medium. Add the remaining 1 tablespoon butter and the thinly sliced sweet yellow onions to the pan drippings with a pinch of salt. Sauté for 10-12 minutes, stirring frequently, until deeply golden brown, caramelized, and fragrant.

Build Rich Beef Gravy: Sprinkle flour over the caramelized onions and stir for 1 minute. Pour in dry white wine or sherry, scraping up the flavorful browned bits from the skillet bottom. Whisk in rich beef bone broth, Dijon mustard, and fresh thyme. Simmer for 2-3 minutes until the gravy turns glossy and slightly thickened.

Simmer Meatballs: Return the seared meatballs and their accumulated juices back into the simmering onion gravy. Reduce heat to low, cover the skillet, and simmer for 6-8 minutes until the meatballs are cooked through to an internal temperature of 165°F (74°C).

Broil the Cheesy Crown: Uncover the skillet and generously scatter shredded Gruyère and Swiss cheese over the meatballs and gravy. Transfer skillet to the oven under the broiler on HIGH for 2-3 minutes until the cheese is bubbling vigorously, melted, and golden-brown blistered.

Garnish & Serve: Scatter fresh thyme leaves over top. Serve immediately right from the hot cast iron skillet with toasted sourdough baguette slices or spooned over creamy garlic mashed potatoes!