In a large bowl, combine ground beef, panko, beaten egg, Parmesan, minced garlic, chopped thyme, Worcestershire sauce, salt, and pepper. Gently mix with hands until just combined without overworking.

Roll mixture into 16 equal-sized meatballs (about 1.5 inches each).

Heat olive oil in a large oven-safe skillet over medium-high heat. Add meatballs and sear for 2 to 3 minutes per side until deeply browned all over. Transfer meatballs to a clean plate (they will finish cooking in the sauce).

Melt butter in the same skillet over medium-low heat. Add sliced onions with a pinch of salt. Cook slowly, stirring frequently, for 20 to 25 minutes until soft, sweet, and deeply golden caramelized.

Sprinkle the flour over the onions and stir continuously for 1 minute to cook out raw flour taste.

Gradually pour in the beef bone broth and Dijon mustard, scraping up all browned bits from the pan bottom. Bring to a gentle simmer for 3 minutes until slightly thickened.

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

Top evenly with shredded Gruyere and mozzarella. Transfer skillet under the oven broiler on high for 3 to 4 minutes until cheese is bubbly and blistered with golden spots.

Garnish with fresh thyme and cracked pepper. Serve immediately with warm crusty baguette slices or mashed potatoes.