Preheat the oven to 350 F. Spray an 8x8-inch baking dish with cooking spray, and set it aside.
In a large bowl, combine the sour cream and cream cheese. Mix together until combined and smooth.
Add the spinach, Parmesan, 1 cup of mozzarella, onion powder, garlic, salt, pepper, and red pepper flakes to the bowl. Blend until the mixture is well-combined.
Transfer the spinach dip to the prepared baking dish and spread it out evenly. Sprinkle the remaining ½ cup of mozzarella cheese on top of the dip.
Bake in the preheated oven for 20 minutes, until the top is golden and the cheese is melted.