Preheat the oven to 350 degrees F. Set oven rack to the middle position.
Spray a baking sheet with baking spray and set aside.
Thaw, drain and squeeze out all the water from the frozen spinach. Set aside.
Bring a large pot of salted water to a boil. Add manicotti shells and cook for 7 minutes (al dente). Drain and rinse with cold water.
Carefully place shells onto the baking sheet.
In a large bowl add ricotta, cream cheese, mozzarella, and parmesan cheeses, eggs, spinach, parsley, salt and pepper. Mix until thoroughly combined.
Transfer cheese and spinach filling to a large pastry bag or large ziploc bag with one corner cut off. Pipe filling into both ends of the manicotti shells, fill until completely full. Repeat until all the shells are filled.
Spread sauce liberally onto the bottom of a 9x13 inch baking dish. Place filled manicotti shells on top of the sauce. Spread sauce liberally over the top of the manicotti shells.
Cover baking dish with foil and bake for 40 minutes. Uncover and sprinkle remaining 1 cup of mozzarella cheese over the top. Bake uncovered for an additional 15-20 minutes until the cheese is melted and bubbly.
Remove from oven and let sit for 10 mintues.
Serve and enjoy!