Preheat the oven to 350°F (175°C). Lightly oil or spread a thin layer of sauce on the bottom of a 9 x 13 baking dish.
Bring a large pot of salted water to a boil and cook the lasagna noodles according to package directions until al dente; drain and lay flat on a sheet of wax paper or clean kitchen towel. Pat dry if needed.
Thoroughly drain the thawed spinach by squeezing in a clean towel or pressing in a colander so no excess liquid remains.
In a medium bowl, combine the drained spinach, ricotta, Parmesan, egg, minced garlic, dried Italian seasoning, diced cooked chicken (if using), and salt and pepper to taste. Mix until evenly distributed.
Spoon about 1 cup of marinara sauce into the prepared baking dish and spread into an even layer.
Place a cooked lasagna noodle on the work surface. Spread about 1/3 cup of the ricotta-spinach mixture evenly over the noodle, leaving a small border. Carefully roll the noodle from one end to the other and place seam-side down in the baking dish. Repeat with remaining noodles and filling.
Spoon remaining marinara sauce over the assembled rolls, then sprinkle each roll with about 1 tablespoon of shredded mozzarella.
Cover the dish tightly with aluminum foil and bake at 350°F (175°C) for 40 minutes, until heated through and cheese is melted. Remove foil for the last few minutes if you want the cheese to brown slightly.
Serve each roll with additional sauce if desired and top with fresh basil if available.