In a large mixing bowl, combine ground beef (or turkey), breadcrumbs, Parmesan cheese, egg, minced garlic, dried Italian herbs, salt, and pepper. Mix until well incorporated. Shape into small meatballs (about 1 inch in diameter).
In a large skillet over medium heat, add a splash of olive oil. Brown the meatballs on all sides for about 8-10 minutes. They don't need to be fully cooked through, as they will finish baking in the oven. Remove from skillet and set aside.
In a pot of boiling salted water, add the cubed potatoes and cook until fork-tender, about 15-20 minutes. Drain well.
Return the potatoes to the pot. Add milk and butter, then mash until smooth and creamy. Season with salt and pepper to taste.
In a greased 9x13 inch baking dish, spread half of the marinara sauce on the bottom. Layer the browned meatballs over the sauce, then spread the mashed potatoes evenly over the meatballs. Spread the remaining marinara sauce over the mashed potatoes.
Sprinkle shredded mozzarella cheese evenly over the top of the bake.
Cover with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let cool for a few minutes. Garnish with freshly chopped parsley before serving.
Notes
Feel free to use ground turkey for a lighter option.