Preheat the oven to 375°F (190°C).

In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, egg, garlic powder, onion powder, oregano, salt, and pepper. Mix thoroughly.

Shape meat mixture into small patties, place a cube of mozzarella in the center, and mold the meat around it to form meatballs. Set aside.

Heat olive oil in an ovenproof skillet over medium heat. Brown meatballs for about 5 minutes, turning occasionally.

Transfer meatballs to a plate. In the same skillet, add butter, sautéed onions for about 5 minutes, then add minced garlic and sauté for 1 more minute.

Sprinkle flour over onion and garlic, stirring continuously for about 2 minutes.

Gradually whisk in beef broth and Guinness stout, stirring until smooth. Let simmer for 4-5 minutes.

Add Worcestershire sauce and return meatballs to the skillet, ensuring they are well coated in gravy.

Transfer the skillet to the oven and bake for 20-25 minutes until meatballs reach an internal temperature of 165°F (74°C).

Let the meatballs rest for 5-10 minutes, garnish with parsley, and serve with sides.