Preheat oven to 350 degrees F.
Steam spinach and drain excess water, pressing with paper towels to remove moisture.
In a mixing bowl, combine greek yogurt, mayonnaise, both cheeses, onion, and garlic powder. Stir to combine.
Spray a baking dish (9x13 or similar) with nonstick cooking spray. Spread half of cheese mixture onto bottom of pan. Top with chicken and spinach. Add rest of cheese mixture on top, spreading evenly.
Optional: sprinkle extra Parmesan cheese on top.
Bake for 40-45 minutes. Optional: Broil for 2 minutes for a crispy, browned top.