In a large mixing bowl, gently combine ground beef, ground pork, panko breadcrumbs, beaten egg, grated cheese, minced garlic, fresh thyme, Worcestershire sauce, salt, and pepper until just combined. Roll into 16 even-sized meatballs.

Heat 2 tablespoons olive oil in a large deep oven-safe skillet over medium-high heat. Sear meatballs for 5 to 6 minutes, turning occasionally until browned on all sides. Transfer meatballs to a clean plate (they will finish cooking in gravy).

Reduce heat to medium-low. Add butter, olive oil, and sliced onions to the skillet with a pinch of salt. Cook slowly for 15 to 18 minutes, stirring frequently and scraping up fond, until deep golden-brown and caramelized. Stir in garlic and thyme for 1 minute.

Sprinkle flour evenly over onions and stir for 2 minutes to cook out raw flour. Deglaze skillet with white wine, scraping up browned bits. Pour in beef bone broth and Worcestershire sauce. Bring to a simmer until slightly thickened (2 to 3 minutes).

Nestle browned meatballs and resting juices back into the bubbling onion gravy. Cover loosely with a lid and simmer gently on medium-low for 8 to 10 minutes until meatballs are thoroughly cooked to 165°F (74°C).

Uncover and scatter shredded Gruyère and Swiss cheeses evenly over meatballs and gravy. Transfer skillet under oven broiler on HIGH for 2 to 3 minutes until cheese is bubbly and golden.

Remove carefully from oven. Garnish with chopped fresh parsley and thyme. Serve immediately with warm crusty baguette or creamy mashed potatoes!