Meatball Casserole

Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with non-stick cooking spray or a thin layer of olive oil.
In a large mixing bowl, combine the ground beef, beaten egg, breadcrumbs, milk, Parmesan cheese, oregano, garlic powder, salt, and pepper. Use your hands to mix gently until all ingredients are just combined, being careful not to overwork the meat.
Shape the meat mixture into 1 1/2-inch meatballs, placing them on a clean plate or baking sheet. You should have approximately 20-24 meatballs.
Heat the olive oil in a large skillet over medium-high heat. Working in batches to avoid crowding, brown the meatballs on all sides for about 5-7 minutes total; they do not need to be cooked through at this stage. Transfer the browned meatballs to the prepared baking dish.
Pour the marinara sauce evenly over the meatballs in the baking dish, gently shaking the dish to distribute the sauce.
Sprinkle the shredded mozzarella cheese evenly over the top of the sauce and meatballs.
Bake uncovered for 20-25 minutes, or until the cheese is melted, bubbly, and lightly browned in spots, and the meatballs are cooked through. Let the casserole rest for 5 minutes before serving. Garnish with chopped fresh parsley.