Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large mixing bowl, whisk together the Greek yogurt, softened cream cheese, minced garlic, onion powder, red pepper flakes, salt, and pepper until uniform.
Fold the chopped artichoke hearts and thoroughly dried spinach into the yogurt mixture.
Add the shredded chicken and half of the mozzarella and Parmesan cheeses to the bowl, stirring until well combined.
Spread the mixture evenly into the prepared baking dish.
Sprinkle the remaining mozzarella and Parmesan cheese over the top of the casserole.
Bake for 30 minutes or until the cheese is golden brown and the edges are bubbling.
Remove from oven and allow to rest for 5 minutes before portioning.