Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Cook spinach according to package directions; drain.
In a skillet over medium heat, cook ground beef until brown; drain. In a large bowl, stir together cooked beef, spinach, pasta sauce and all but 1/4 cup of the Cheddar cheese. Reserve 2 cups of sauce mixture. Stir the pasta shells into the remaining sauce
mixture. Pour shell mixture into a 9x13 inch baking dish and top with reserved sauce and sprinkle with reserved cheese.
Bake in preheated oven 45 minutes, until browned and bubbly.