Preheat the oven to 350°F (175°C). Lightly spread about 1 cup of marinara sauce over the bottom of a 9x13-inch 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 noodles flat on wax or parchment paper; pat dry with paper towels.
Make sure the thawed spinach is very well drained by pressing in a colander or squeezing in a clean towel, then chop or fluff if needed.
In a mixing 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 combined.
Spoon about 1/3 cup of the ricotta-spinach mixture onto each cooked noodle and spread into an even layer. Roll each noodle up tightly and place seam-side down in the prepared baking dish.
Ladle remaining marinara sauce over the rolls so they are coated, then sprinkle 1 tablespoon of shredded mozzarella over each roll.
Cover the dish tightly with aluminum foil and bake for about 40 minutes, until heated through and cheese is melted. Remove foil for the last few minutes if you want the cheese lightly browned.
To serve, spoon a little sauce onto each plate and place a lasagna roll on top; garnish with fresh basil if desired.
