Preheat your oven to 375°F (190°C) and lightly grease an 8x8-inch baking dish.
In a large mixing bowl, thoroughly combine the Fage Total 2% Greek Yogurt, minced garlic, Parmesan cheese, onion powder, salt, and pepper until uniform.
Fold in the fresh baby spinach and cooked chicken breast, ensuring everything is well coated with the yogurt mixture.
Transfer the mixture into the prepared baking dish and smooth down the top with a spatula.
Bake uncovered for 15-20 minutes until the casserole is heated all the way through and bubbling at the edges.