Preheat the oven to 350°F (175°C). Spread about 1 cup of marinara sauce over the bottom of a 9 x 13-inch baking dish.
Cook the lasagna noodles in a large pot of boiling salted water according to package directions until al dente; drain and lay flat on wax paper or a clean towel. Pat noodles dry.
Make sure the thawed spinach is well drained, squeezing or pressing out excess moisture in a clean towel or colander.
In a medium bowl, combine the drained spinach, ricotta, grated Parmesan, egg, minced garlic, dried Italian seasoning, diced cooked chicken (if using), and salt and pepper to taste; mix until evenly combined.
Spoon about 1/3 cup of the spinach-ricotta mixture evenly onto each noodle and spread into a strip, leaving short ends free. Roll each noodle tightly and place seam-side down in the prepared baking dish.
Ladle the remaining marinara sauce over the rolls, then sprinkle about 1 tablespoon of shredded mozzarella over each roll.
Cover the baking dish tightly with aluminum foil and bake for 40 minutes, or until heated through and the cheese is melted.
Serve by spooning a little sauce on each plate, topping with a lasagna roll, and garnishing as desired.