Preheat your oven to 350°F (175°C).
Cook the manicotti shells according to the package instructions until al dente. Drain the shells and rinse with cold water to prevent them from sticking together. Set aside.
In a large mixing bowl, combine the ricotta cheese, 1½ cups of shredded mozzarella, ¾ cup of grated Parmesan, egg, parsley, salt, and pepper. Mix until well combined.
Carefully stuff each cooked manicotti shell with the cheese mixture using a spoon, piping bag, or a small plastic bag with a corner snipped off.
Spread about 1 cup of marinara sauce evenly across the bottom of a 9x13-inch (23x33 cm) baking dish. Place the filled manicotti shells in the baking dish in a single layer.
Pour the remaining 1½ cups of marinara sauce over the top of the stuffed manicotti shells, making sure they are fully covered. Sprinkle the remaining ½ cup of shredded mozzarella and ¼ cup of grated Parmesan cheese over the top of the manicotti. If desired, add grated Pecorino Romano cheese as well.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and continue baking for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Remove the manicotti from the oven and let it rest for 5 minutes before serving. Garnish with additional parsley, if desired.