Heat the oven to 400°F (200°C). Line a rimmed baking sheet with parchment or foil and lightly coat it with nonstick spray or 1 teaspoon olive oil.

In a large bowl, combine the breadcrumbs, 1/3 cup Parmesan, egg, milk, garlic, parsley, Italian seasoning, kosher salt, and black pepper. Stir until the mixture is evenly moistened.

Add the ground beef and mix gently with your hands or a fork just until the ingredients come together. Do not compact or overmix the meat, because that can make the finished meatballs dense.

Shape the mixture into 20–24 meatballs, each about 1 1/2 inches in diameter. A small cookie scoop helps create even portions, and the meatballs should be arranged with a little space between them on the prepared sheet.

Bake for 16–20 minutes, until the meatballs are browned and cooked through. Check that the center reaches 160°F (71°C), especially when the meatballs vary slightly in size.

Spread about half of the marinara sauce in a baking dish. Transfer the baked meatballs into the dish and spoon the remaining sauce over the top so each one is coated.

Sprinkle the mozzarella and 2 tablespoons grated Parmesan over the sauced meatballs. Return the dish to the oven for 5–8 minutes, until the cheese is melted and bubbling.

Rest the dish for 5 minutes before serving, then garnish with parsley or basil. If you want more color, broil briefly after baking, watching closely so the cheese does not burn.