Cook the manicotti according to the instructions on the box and set aside separately to bring back to room temperature.
Add olive oil to a large pan over medium high heat. Once melted, add garlic and cook for 30-60 seconds making sure not to overcook and cause burning.
Add heavy cream, ½ cup of parmesan cheese, ½ tablespoon of cajun seasoning, Italian Seasoning, pinch of red pepper flakes, and pinch of ground nutmeg and stir to mix together. Bring cream sauce to a boil then reduce heat to low and simmer for 5-7 minutes to thicken. Once thickened, remove from heat and pour into a large baking pan and set aside.
Next in a separate medium sized bowl, add ricotta cheese, spinach, onion, remaining parmesan and remaining cajun seasoning and stir together to combine well.
Stuff room temperature manicotti with spinach mixture and add to the bottom of the sauced baking pan.
Cover with mozzarella cheese and bake for 20 minutes or until the cheese has completely melted and is bubbling. Remove from the oven. Allow to cool for 10-15 minutes, garnish with parsley and serve.
To store leftovers, transfer the manicotti to an airtight container or cover the original baking dish with plastic wrap. The leftovers will keep for 3-5 days refrigerated.For easy reheating, place your desired portion on a microwave-safe dish or plate and microwave at 30 second intervals until heated through. Alternatively, you can reheat the leftovers in the original baking dish in the oven for 15-20 minutes.