Preheat your oven to 375°F (190°C).
In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic, salt, and black pepper. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter, and place them on a baking sheet.
Bake the meatballs in the preheated oven for 20 minutes, or until they are cooked through and browned.
In a large casserole dish, spread half of the marinara sauce on the bottom.
Arrange the baked meatballs over the sauce in a single layer.
Pour the remaining marinara sauce over the meatballs, then sprinkle the mozzarella cheese on top.
Sprinkle Italian seasoning and red pepper flakes over the cheese, if using.
Cover the casserole dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the casserole cool for a few minutes before serving. Garnish with fresh basil leaves if desired.