In a large bowl, combine the egg, panko, onion, parsley, garlic powder, salt, pepper, allspice and nutmeg. Mix well.
Add the ground beef to the bowl and use your hands to mix until just combined. Try not to over-mix or the meatballs may turn out tough.
Heat the olive oil and 1 tablespoon of the butter in a large nonstick skillet, such as a cast iron skillet, or a Dutch oven over medium heat. Use your hands to shape the meat mixture into 1 ¼-inch meatballs and place them in a single layer in the pan. (You should get about 22 meatballs.)
Cook, gently turning the meatballs so that they brown evenly on all sides, until cooked through to 165° F. Remove meatballs to a clean plate.
Add the remaining 2 tablespoons butter to the pan. Once melted, whisk in the flour and cook for 1-2 minutes. Slowly whisk in the beef broth.
Whisk in the the Dijon, salt and pepper, followed by the heavy cream. Simmer the sauce until thickened, whisking occasionally, about 2-3 minutes. Then add the meatballs back to the pan and coat with the sauce. Simmer the meatballs in the sauce for 2-3 minutes, stirring occasionally, until heated through.
Serve meatballs and sauce over egg noodles, mashed potatoes or rice, garnished with chopped fresh parsley.