Preheat the oven to 350°F.
In a bowl, add ricotta cheese, shredded mozzarella, parmesan cheese, chopped spinach, egg, salt and pepper. Mix until fully combined.
Transfer the creamy filling into a piping bag. (You can also just use a spoon to stuff the shell, but to make it cleaner and faster use a piping bag.)
In a 7.5x6 casserole dish, add half of the marinara sauce to the bottom of the dish, spreading evenly.
Stuff the shells with the ricotta filling by piping (or spooning) about 2-3 Tbsp of the mixture.
Add the stuffed shells to the dish, over the meat sauce, arranging flatly.
Once all the shells are stuffed and added to the dish, spread the remaining pasta meat sauce over the stuffed shells. Top with shredded cheese and cover the dish with foil.
Bake at 350°F covered in foil for 35 minutes. Remove the foil and bake for an additional 5-10 minutes, until bubbly and crispy along the edges.
Remove from oven and garnish with additional chopped greens and serve immediately.