Preheat the oven to 350°F (175°C). Pour about 1 cup of marinara sauce into the bottom of a 9 x 13-inch casserole dish and spread evenly.
Boil the lasagna noodles according to package directions until al dente, then drain and lay them flat on wax paper or a baking sheet; pat dry with paper towels.
Thoroughly drain the thawed spinach by squeezing out excess moisture, then chop if pieces are large.
In a medium bowl, combine drained spinach, ricotta, grated Parmesan, egg, minced garlic, dried Italian seasoning, cooked diced chicken (if using), and salt and fresh pepper to taste until evenly mixed.
Spoon about 1/3 cup of the ricotta-spinach mixture onto one end of each lasagna noodle and spread it into an even layer, leaving a small border; carefully roll the noodle up and place seam-side down in the prepared baking dish. Repeat for all noodles.
Ladle the remaining marinara sauce over the assembled rolls, then sprinkle 1 tablespoon of shredded mozzarella on top of each roll.
Cover the dish tightly with aluminum foil and bake for 40 minutes, or until heated through and the cheese has melted.
To serve, place a little sauce on each plate and top with a lasagna roll; garnish with fresh basil if desired.