In a large mixing bowl, whisk together the panko breadcrumbs, milk, and beaten egg. Let sit for 3 minutes so the breadcrumbs absorb the liquid into a soft panade (this is the chef secret to tender, juicy meatballs). Add the ground beef, minced garlic, grated Parmesan, fresh thyme, Worcestershire sauce, kosher salt, and cracked black pepper. Gently mix with clean hands until just evenly incorporated—avoid overworking the meat.

Shape the meat mixture into 12 to 14 equal-sized meatballs (about 1.5 inches each). Heat 1 tablespoon olive oil in a 12-inch heavy cast-iron or stainless steel skillet over medium-high heat. Add the meatballs in a single layer and sear undisturbed for 2 to 3 minutes, then gently turn to brown all sides (about 5 to 6 minutes total). They do not need to be cooked through yet. Transfer seared meatballs to a clean plate and set aside.

Lower the heat to medium. Add 2 tablespoons unsalted butter to the same skillet along with the thinly sliced sweet yellow onions and a pinch of salt. Cook slowly, stirring often and scraping up browned bits, for 12 to 15 minutes until the onions turn soft, translucent, and deeply golden caramel-brown.

Sprinkle 1 tablespoon all-purpose flour over the caramelized onions and stir constantly for 60 seconds to toast the flour. Slowly pour in the beef bone broth while whisking or stirring to dissolve any remaining fond from the bottom of the pan. Stir in the heavy cream, Dijon mustard, Worcestershire sauce, and thyme sprigs. Bring the sauce to a gentle simmer for 2 minutes until it turns glossy and begins to coat the back of a spoon.

Gently return the meatballs and any resting juices back into the simmering onion gravy. Spoon the rich gravy generously over the meatballs. Reduce heat to medium-low, partially cover, and simmer for 5 to 6 minutes until the meatballs are thoroughly cooked through and register 165°F (74°C) on an instant-read thermometer.

Remove and discard the thyme sprig stems. Generously sprinkle 1 cup of freshly grated Gruyere cheese across the meatballs. Cover the pan with a lid for 2 minutes until molten and gooey, or place the oven-safe skillet under your oven broiler on HIGH for 2 to 3 minutes until bubbling with toasted golden spots. Garnish with fresh thyme and serve immediately over warm mashed potatoes or with crusty artisan rolls.