Get a large pot of water boiling with a dash of salt. When it comes to a full boil, add the lasagna noodles and cook until soft but not soggy, about 12-15 minutes. When they are finished cooking, drain in a colander.
While the noodles are boiling, prepare the filling. Thaw the package of frozen spinach in the microwave and then squeeze out as much excess liquid as possible. Combine the spinach in a bowl with the ricotta, mozzarella, parmesan, egg, freshly grated pepper and about ¼ tsp of salt. Mix until well combined.
When the noodles and filling are ready to go, preheat the oven to 400 degrees. Prepare a glass casserole dish by spraying with non-stick spray.
On a clean surface, lay out a few noodles at a time. Place a few tablespoons of filling on each noodle and spread to cover from edge to edge. The filling does not need to be thick because once the noodle is rolled up, it will be compounded. Make sure to spread it all the way to the edges of the noodles.
Roll the noodles up and place in the prepared casserole dish. Repeat until all of your filling is gone (there may be some noodles left over, these are “back ups” in case any of the others rip during assembly). Pour the marinara sauce over the rolled noodles making sure to cover all surfaces. The sauce will keep the noodles hydrated and soft while baking.
Cover the dish in foil and bake for 30 minutes. Serve hot or divide into individual portions and refrigerate.