In a medium-sized bowl, combine the ground beef, panko breadcrumbs, parsley, allspice, nutmeg, chopped onion, garlic powder, pepper, salt, and egg. Mix thoroughly until the ingredients are well incorporated.

Shape the mixture into 12 large meatballs or 20 smaller ones. In a large skillet, heat olive oil and 1 tablespoon of butter over medium heat. Add the meatballs and cook them, turning occasionally, until they are golden brown on all sides and fully cooked through. Remove the meatballs and set them aside, covering them with foil to keep warm.

To make the sauce, melt the remaining butter in the same skillet. Add the flour and whisk until the mixture turns a golden brown color. Gradually stir in the beef broth and heavy cream, followed by Worcestershire sauce and Dijon mustard. Bring the mixture to a simmer, allowing it to thicken. Season with salt and pepper to taste.

Return the meatballs to the skillet, gently stirring them into the gravy. Simmer for an additional 1-2 minutes, allowing the meatballs to absorb the rich sauce. Serve immediately, ideally over a bed of egg noodles or rice for a complete meal.