Bring a large pot of salted water to a boil. Add the jumbo pasta shells and cook until just al dente—about 8–9 minutes, or according to package directions. The shells should be tender but firm enough to hold their shape when filled.
Drain the shells and spread them out on a clean tray or large plate to cool slightly. This prevents them from sticking together and makes them easier to handle during assembly.
Prepare the Filling
In a large mixing bowl, combine the shredded or diced cooked chicken, ricotta cheese, shredded mozzarella, Parmesan, Cajun seasoning, and minced garlic. If using spinach, chop it finely and mix it in as well.
Season with salt and pepper to taste, then stir the mixture thoroughly until smooth and evenly combined. The filling should be creamy and cohesive, making it easy to spoon into the shells without falling apart.