Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the diced onion and minced garlic. Cook until soft, then remove from heat and set aside.

In a large mixing bowl, combine the panko breadcrumbs, Parmesan, beaten egg, milk, salt, pepper, parsley, oregano, allspice, nutmeg, and the cooled onion-garlic mixture. Add the ground beef and ground pork. Gently mix until just combined.

Shape the meat mixture into 1 1/2-inch meatballs and place them on a plate. Chill the meatballs in the refrigerator for about 15 minutes to help them hold their shape.

Heat the remaining tablespoon of olive oil in the skillet over medium-high heat. Brown the meatballs in batches, about 1 minute per side, until they are evenly browned. Remove the meatballs and set aside.

In the same skillet, melt the butter over medium heat. Stir in the flour and cook for 2 minutes while stirring constantly. Gradually whisk in the beef broth until the mixture is smooth. Add the Worcestershire sauce and Dijon mustard. Stir in the sour cream and mix well.

Return the browned meatballs to the skillet. Simmer over low heat for 10 to 15 minutes until the meatballs are heated through and the gravy has thickened (if it becomes too thick, add a splash or two of broth). Garnish with chopped parsley. Serve the meatballs with your favorite side, such as egg noodles or mashed potatoes.