Preheat oven to 350 F. Cook pasta according to package directions, drain and set aside. Thaw the frozen spinach according to the package directions.
Spread a thin layer of marinara sauce over the bottom of a large baking dish. In a medium bowl, combine ricotta cheese, thawed spinach, grated parmesan cheese, minced garlic and 1/2 cup shredded mozzarella cheese.
Stuff each pasta shell with the spinach ricotta cheese mixture. Place the pasta shells into the baking dish. Cover with the remaining marinara sauce and sprinkle with the remaining mozzarella cheese.
Cover with foil and bake for 20 minutes. Uncover and bake for another 10-15 minutes, until the cheese has melted.
