Cook manicotti pasta shells for two minutes less than the package directions. Rinse pasta with cool water and drain.
Heat oven to 350 degrees.
In a small skillet, brown 1 lb of Italian Sausage over medium heat. Add a jar of store-bought marinara sauce and simmer on low heat for approximately 10 minutes. Remove from heat and set aside.
In a large skillet, wilt 5.5 ounces of baby spinach (about 5 cups). Move spinach around with tongs continuously until all the spinach is wilted. This should only take a few minutes.
To make the filling, add 1 cup ricotta cheese, ½ cup whipped cream cheese, ½ cup Parmesan cheese, ¼ tsp salt, ½ tsp pepper, 2 tsp Italian seasoning, 1 egg, and the wilted spinach to a small bowl and mix until the ingredients are incorporated.
To a piping bag add the cheese mixture. Cut a large hole at the bottom and fill each cooked manicotti shell.
In a rectangular 9 x 13 in baking dish add 1 cup of the sausage mixture to the bottom of the pan. Next, arrange the filled manicotti shells in a single layer. You’ll then add the remainder of the sausage mixture to the top of the manicotti and for the final step sprinkle 1 cup of mozzarella cheese over the top of the entire dish. Cover with foil and bake for 30 minutes.
Remove foil and bake an additional 5 minutes until the cheese begins to brown. Remove from oven and let rest for 5 minutes before serving.