• Preheat the oven to 350° F and spray a 7x11 baking dish with non-stick cooking spray.
• Bring a large pot of water to a boil over medium high heat.
• Once water boils, cook manicotti for about 7 minutes, drain.

For the Italian Sausage Mixture

• In a large frying pan over medium high heat, cook sausage and onions until sausage is no longer pink and onions are translucent. Drain grease and put mixture into a large bowl.
• Add the thawed, drained spinach to the meat mixture. Stir to combine.

For the Ricotta Filling

• In a medium bowl, mix together ricotta, parmesan, egg, and dried parsley flakes.
• Add ricotta mixture into the sausage mixture and stir until everything is mixed well.

For the Tomato Sauce

• In the pan used to cook the sausage, add in tomato sauce, crushed tomatoes, tomato paste and water. Stir together and add in seasonings.
• Reduce heat and bring sauce to a simmer.
• Pour ½ cup of sauce into the bottom of the prepared dish.
• Fill each manicotti noodle with the ricotta meat mixture and place in the prepared baking dish. Repeat until all noodles are filled and placed in the dish.
• Pour remaining sauce over the stuffed noodles and then top with shredded mozzarella cheese.
• Cover the dish with foil and bake in the oven for 35-40 minutes.