In a large mixing bowl, gently combine ground chicken, panko, parmesan, beaten egg, minced garlic, parsley, salt, and black pepper until just incorporated (avoid overmixing). Roll into 16 evenly sized meatballs.

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

Lower skillet heat to medium. Add butter and remaining olive oil. Add the thinly sliced onions with a pinch of salt. Sauté slowly for 12 to 15 minutes, stirring frequently, until the onions soften, collapse, and turn a deep caramelized amber brown.

Sprinkle flour over the caramelized onions and stir continuously for 1 minute. Pour in the beef bone broth and Worcestershire sauce while scraping up every flavorful browned bit from the bottom of the skillet. Add the fresh thyme sprigs and bring to a gentle simmer for 3 minutes until slightly thickened.

Nestle the seared chicken meatballs back into the bubbling onion gravy. Cover the skillet and simmer over medium-low heat for 6 to 8 minutes until the meatballs reach an internal temperature of 165°F (74°C).

Remove the lid and discard the woody thyme stems. Generously blanket the meatballs with shredded mozzarella and Swiss cheese. Cover for 2 minutes to melt, or slide the skillet under your oven broiler for 2 to 3 minutes until the cheese is bubbling, golden, and blistered. Garnish with fresh thyme and serve warm.