Heat the large skillet over medium-high heat and add the ground beef, breaking it apart as it cooks.

Cook for about 5-7 minutes until the beef is fully browned and no pink remains, then drain any excess fat in a colander.

In the medium mixing bowl, combine the cooked ground beef, ricotta cheese, 1 cup mozzarella, Parmesan, egg, salt, pepper, garlic powder, red pepper flakes, and fresh parsley if using.

Stir until everything is well combined and the mixture holds together.

Fill the large pot with water and bring it to a rolling boil, then add salt.

Add the manicotti tubes and cook according to package directions, usually 8-10 minutes, until just barely tender but still firm.

Drain the pasta carefully and spread the tubes on a clean kitchen towel to cool slightly.

Preheat the oven to 375 degrees Fahrenheit.

Pour about 1/2 cup of marinara sauce into the bottom of the baking dish and spread it evenly.

Using a piping bag, carefully fill each manicotti tube with the meat and cheese mixture.

Place the filled manicotti tubes seam-side up in a single layer over the sauce in the baking dish.

Pour the remaining marinara sauce evenly over the tubes.

Sprinkle the remaining 1/2 cup mozzarella and 2 tablespoons Parmesan over the top.

Cover the baking dish tightly with aluminum foil and bake at 375 degrees Fahrenheit for 25 minutes.

Remove the foil and bake uncovered for an additional 15-20 minutes until cheese is melted and bubbly.

Let the manicotti rest for 5 minutes before serving.