Steam the spinach until it is wilted. Allow to cool and squeeze out as much water as you can. Chop the spinach.
In a bowl, mix the cream cheese, sour cream, chopped spinach, garlic, salt, pepper, half of the Parmesan cheese, and half of the mozzarella cheese.
Transfer the mixture into an oven-safe dish, then sprinkle with the rest of the mozzarella and Parmesan. Pop it into the oven, and bake for 20-25 minutes, or until the cheese is melted and the top is bubbling and browned.