Make the Meatballs:
Preheat your oven to 375°F (190°C).
In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, milk, egg, garlic, Italian seasoning, salt, and pepper. Mix until well combined.
Roll the mixture into 1 to 1.5-inch meatballs and place them on a parchment-lined baking sheet.
Heat the olive oil in a large skillet over medium heat. Brown the meatballs in batches, about 3-4 minutes per side. Once browned, remove them from the skillet and set aside.

Cook the Mashed Potatoes:
In a large pot, boil the chopped potatoes in salted water for 12-15 minutes, or until fork-tender. Drain the potatoes and return them to the pot.
Add the butter and milk to the potatoes, and mash until smooth and creamy. Season with salt and pepper to taste. If you want extra creaminess, add the shredded cheddar cheese into the mashed potatoes and mix until melted and combined.

Assemble the Casserole:
Spread the marinara sauce in the bottom of a greased 9×13-inch baking dish.
Arrange the browned meatballs evenly over the sauce.
Spoon the mashed potatoes over the meatballs, spreading it evenly to cover the entire dish.
Top with the shredded mozzarella cheese.

Bake:
Bake the casserole in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly and the edges are golden brown.

Serve:
Let the casserole cool for a few minutes before serving. Garnish with fresh parsley if desired. Serve with a side salad or some steamed vegetables for a complete meal.