Preheat oven to 400°F. Spray baking dish with vegetable oil cooking spray and coat the bottom of the baking dish with 1 cup marinara sauce. Place one chicken breast between 2 sheets of plastic wrap on a cutting board. Roll and pound with a rolling pin until meat is about 1/4-inch thick. Repeat with remaining chicken breasts. Press 1/2 cup spinach leaves on each chicken breast. Place 1 slice mozzarella cheese on each and roll up tightly, pressing firmly. Place each chicken roll seam side down in baking dish and cover with remaining marinara. Cover with aluminum foil and bake at 400°F for 35 minutes. Remove aluminum foil and continue baking at 400°F for 10 minutes. Garnish with grated Parmesan cheese and red pepper flakes to taste.