Heat oven to 400 degrees. Cook the pasta according to the package directions; drain and run under cold water until cool.
Spread the marinara sauce in the bottom of a large broiler-proof baking dish.
In a large bowl, combine the ricotta, spinach, Parmesan, 1/2 tsp salt, 1/4 tsp pepper. Spoon the mixture into the shells and place them on top of the sauce. Sprinkle with mozzarella and bake until the shells are heated through, 10-12 minutes.
Increase heat to broil. Broil shells until the cheese beings to brown, 2-5 minutes
Serve with either homemade or store bought marinara sauce..