In a large mixing bowl, gently combine ground beef, panko breadcrumbs, beaten egg, minced garlic, parsley, Worcestershire sauce, salt, and pepper until just incorporated. Shape into 16 plump, uniform meatballs.
Heat 1 tbsp olive oil in a 12-inch oven-safe cast iron skillet over medium-high heat. Sear the meatballs for 5 to 6 minutes, turning gently, until browned on all sides. Transfer meatballs to a warm plate (they will finish cooking in the gravy).
In the same skillet, melt butter with remaining 1 tbsp olive oil over medium-low heat. Add the sliced yellow onions and a pinch of salt. Cook low and slow for 20 to 25 minutes, stirring occasionally, until soft, jammy, and deeply caramelized to a golden amber hue.
Sprinkle flour over the caramelized onions and stir constantly for 1 minute to cook out the raw flour taste. Pour in beef bone broth and fresh thyme leaves, using a wooden spoon to scrape up all the delicious browned bits (fond) from the bottom of the skillet. Simmer for 4 to 5 minutes until the gravy thickens slightly.
Nestle the seared meatballs and any resting juices back into the bubbling onion gravy. Spoon gravy generously over each meatball, reduce heat to low, cover, and simmer for 8 minutes until meatballs reach an internal temperature of 160 F (71 C).
Uncover and blanket the meatballs with shredded Gruyu00e8re and Swiss cheese. Broil in the oven (or cover with a lid over low heat) for 3 to 4 minutes until cheese is completely melted, bubbling, and blistered with golden-brown spots.
Garnish with fresh thyme leaves and cracked black pepper. Serve immediately piping hot with creamy mashed potatoes, buttered egg noodles, or warm crusty French baguette.