In a large mixing bowl, combine the ground beef, panko breadcrumbs, beaten egg, grated parmesan, minced garlic, chopped parsley, onion powder, sea salt, and black pepper. Mix gently with your hands just until combined—avoid overworking to ensure tender, melt-in-your-mouth meatballs.

Roll the beef mixture into 16 even-sized meatballs (about 1.5 inches in diameter). Place them on a plate and set aside.

Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a large deep cast iron skillet over medium-high heat. Once hot and shimmering, add the meatballs in a single layer. Sear undisturbed for 2–3 minutes per side until deeply golden-brown all around (about 6–8 minutes total). Transfer seared meatballs to a warm plate (they will finish cooking in the gravy).

Lower heat to medium. Add the remaining 2 tablespoons of butter and minced garlic to the skillet drippings. Sauté for 45–60 seconds until fragrant, stirring constantly to avoid scorching the garlic. Sprinkle the flour over the garlic butter and whisk for 1 minute to cook out raw flour taste.

Slowly pour in the beef bone broth while scraping up all the caramelized golden browned bits (fond) from the bottom of the pan with a wooden spoon. Whisk in the heavy cream and Dijon mustard. Bring the sauce to a gentle simmer and let it bubble for 2–3 minutes until slightly reduced and glossy.

Stir in the grated parmesan cheese until silky and fully melted. Return the browned meatballs and any resting juices back into the skillet. Spoon the bubbling garlic butter gravy over the meatballs, reduce heat to medium-low, and simmer gently for 4–5 minutes until the meatballs are thoroughly cooked through and coated in glossy sauce.

Garnish generously with fresh flat-leaf parsley and cracked black pepper. Serve immediately over creamy mashed potatoes, buttered egg noodles, or with warm crusty artisan bread.