Preheat the oven to 375°F (190°C).
In a mixing bowl, combine all meatball ingredients: ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic, salt, pepper, and oregano. Mix well until combined.
Form meat mixture into 1-inch meatballs and place them on a baking sheet. Bake in the preheated oven for 20 minutes or until cooked through.
While the meatballs are baking, prepare the mashed potatoes. In a medium pot, boil the quartered potatoes in salted water until tender, about 15 minutes. Drain the potatoes.
Add milk, butter, and salt and pepper to the drained potatoes. Mash until smooth and creamy.
Once the meatballs are cooked, spread the marinara sauce evenly in the bottom of the large baking dish.
Arrange the baked meatballs over the sauce.
Spread the mashed potatoes generously over the meatballs to create an even layer.
Sprinkle the shredded mozzarella cheese on top of the mashed potatoes.
Bake the assembled casserole in the oven for an additional 25 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes before serving.