In a large mixing bowl, combine the ground beef, breadcrumbs, grated Parmesan cheese, egg, garlic powder, onion powder, dried Italian herbs, salt, and pepper. Mix gently until just combined, being careful not to overwork.
Shape the mixture into small, evenly sized meatballs about 1 inch in diameter. Heat a non-stick skillet over medium heat with a tablespoon of oil. Cook the meatballs in batches, turning occasionally, until browned and cooked through, about 8-10 minutes. Remove from heat.
Prepare mashed potatoes if not already done by boiling peeled potatoes until tender, then mashing with butter, milk, salt, and pepper until smooth and creamy. Leftover mashed potatoes can also be used.
Preheat oven to 375°F (190°C). In a baking dish, spread half the mashed potatoes evenly as the first layer. Arrange half the cooked meatballs over the potatoes. Spoon half the marinara sauce on top, then sprinkle half the shredded mozzarella cheese.
Repeat the layers with the remaining mashed potatoes, meatballs, marinara sauce, and mozzarella cheese.
Bake in the preheated oven for 20-25 minutes until the cheese is melted, bubbly, and slightly golden.
Remove from oven, let cool for a few minutes. Garnish with freshly chopped parsley. Serve warm and enjoy.