Preheat the oven to 400 degrees . Bring a large pot of salted water to a boil. Add a splash of EVOO and the lasagna noodles. Cook until al dente. Drain, then rinse with cold water, being careful not to tear the noodles. Transfer the noodles to a large bowl of cold water until ready to use.
Rinse out the pot, add 1 inch of water and bring to a boil. Add the spinach and cook, stirring, until wilted, 1 to 2 minutes. Drain and rinse with cold water. Using your hands, squeeze all the liquid from the spinach.
Pulse the spinach, ricotta, egg, Parmesan and pepper in a food processor to blend. Transfer the mixture to a medium bowl.
Grease a 9-inch square baking dish. Spread 1 3/4 cups of the marinara sauce in the dish.
Place 4 noodles on a work surface and spread each with 1/4 cup of the spinach-ricotta mixture. Place a piece of string cheese horizontally across one end of each noodle. Roll up the noodle around the cheese.
Cut each pinwheel in half crosswise with a serrated knife. Arrange pinwheels, cut side down, in the baking dish. Repeat with the remaining 4 lasagna noodles, spinach- ricotta mixture and string cheese.
Cover the dish tightly with foil. Bake until the sauce is bubbling and the spinach-ricotta mixture is firm, about 45 minutes. Let stand 5 minutes.
Warm the remaining 3/4 cup marinara sauce in a small saucepan over medium heat. Spoon some sauce onto each plate. Top with pinwheels.