
Preheat the oven to 350 F.
Bring a large pot of water to a boil. Boil the manicotti noodles according to package directions, then drain and set aside.
In a medium-sized bowl, add the ricotta cheese, egg, and Parmesan cheese. Using a spoon, mix well. Scoop the filling into a large ziplock bag, pushing the air out. Snip a corner off the bottom of the bag to make a piping bag.
In a 9x13-inch baking dish, add ½ the jar of marinara sauce.
Pipe the ricotta mixture into each of the 8 noodles. Set the filled noodles into the baking dish. Repeat with all noodles.
Pour the remaining sauce over the noodles.
Sprinkle mozzarella cheese over top of the sauce-covered noodles.
Bake for 30 minutes until the cheese is melted, then serve.