Set an oven rack to the highest position and heat oven to 400° F. Cook the pasta according to the package directions. Drain and run under cold water to cool.
Spread the marinara sauce in the bottom of a large broilerproof baking dish.
In a bowl, combine the ricotta, spinach, Parmesan, ½ teaspoon salt, and ¼ teaspoon pepper. Spoon the mixture into the shells and place them on top of the sauce.
Sprinkle the shells with the mozzarella and bake until heated through, 10 to 12 minutes.
Increase heat to broil. Broil the shells until cheese begins to brown, 2 to 5 minutes. Serve with the salad, if desired.