Preheat oven to 375°F (190°C). Lightly grease a baking dish.
Heat olive oil in a skillet over medium heat and sauté garlic until fragrant, about 1 minute.
In a large bowl, combine the chicken, spinach, ricotta, sour cream, mozzarella, Parmesan, salt, pepper, and nutmeg.
Spread the mixture evenly into the prepared baking dish.
Sprinkle the top with remaining mozzarella and Parmesan cheese.
Bake uncovered for 25 to 30 minutes, or until bubbly and golden on top.
Let cool slightly before serving.