Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish. This prevents sticking and ensures an even bake.
In a large bowl, whisk together sour cream, heavy cream, garlic powder, salt, black pepper, and paprika until smooth. This creamy mixture will coat the chicken and spinach perfectly.
Spread the chopped spinach evenly in the greased baking dish, followed by the shredded chicken. Pour the prepared creamy sauce over the top, ensuring everything is well covered.
Sprinkle mozzarella and Parmesan cheese generously over the casserole. Cover with foil and bake for 20 minutes. Then, remove the foil and bake for another 10-15 minutes until the cheese is golden and bubbly.