Preheat oven to 350°F. Spray a 9†x 9†baking dish with Non-Stick Cooking Spray.
Combine Light Mayonnaise, sour cream, garlic powder, pepper, and Parmesan cheese and shredded mozzarella in a medium bowl.
In another bowl, toss 1/3 of the sauce with the chicken and spinach and spread in the casserole dish. Pour the rest of the sauce mixture over the chicken and spinach.
Bake, uncovered, 40 minutes or until top is golden brown.
Swapping the Base Fats
Replacing the light mayonnaise with full-fat Greek yogurt alters the moisture retention during baking. Yogurt contains less oil and more protein, which causes the binding sauce to tighten faster under dry oven heat. Adjusting this dairy component requires monitoring the internal temperature closely so the poultry proteins do not seize and release excess water into the dish.
Managing Moisture and Mixing
Frozen spinach retains significant cellular water even after pressing by hand. Squeezing the thawed greens inside a clean kitchen towel until completely dry prevents the excess liquid from diluting the mayonnaise and sour cream binder. Tossing one-third of the cheese sauce directly with the diced chicken and spinach coats the individual pieces before the remaining blanket is poured over the top. This dual-stage coating ensures the ingredients bond uniformly rather than separating into distinct layers during the 40-minute bake at 350°F.
Understanding Second-Day Changes
Casseroles containing poultry and dairy develop deeper flavors after resting in refrigeration. Storing leftovers in a sealed glass container for up to 48 hours allows the garlic powder and grated parmesan to fully hydrate into the surrounding proteins. Reheating individual portions gently at 300°F prevents the fats in the mozzarella and mayonnaise from breaking and oiling off on the surface.