Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, parsley, egg, minced garlic, onion powder, salt, black pepper, and oregano. Mix until well combined.

Form the meat mixture into meatballs, roughly 1 inch in diameter. Place the meatballs on a baking sheet lined with parchment paper.

Bake the meatballs in the preheated oven for about 20 minutes, or until they are cooked through and browned.

While the meatballs are baking, prepare the mashed potatoes if you haven't already. In a large bowl, combine the prepared mashed potatoes with sour cream and milk, mixing until smooth and creamy.

In a greased 9x13 inch baking dish, spread half of the marinara sauce on the bottom. Layer the baked meatballs evenly over the sauce.

Spoon the mashed potatoes over the meatballs, spreading them evenly to cover. Drizzle the remaining marinara sauce over the mashed potatoes.

Sprinkle shredded mozzarella cheese generously over the top.

Bake in the oven for an additional 25-30 minutes, or until the cheese is bubbly and golden brown.

Remove from the oven and let it rest for a few minutes before serving.