Preheat your oven to 350°F. Grease a 9x13-inch baking dish with butter or cooking spray.
In a large mixing bowl, combine the squeezed-dry spinach, cream cheese, sour cream, 1 cup of mozzarella cheese, Parmesan cheese, minced garlic, onion powder, salt, and pepper. Mix until well combined.
Spread the spinach mixture evenly into the prepared baking dish.
In a small bowl, mix together the panko breadcrumbs, remaining 1/2 cup mozzarella cheese, and melted butter. Sprinkle this mixture evenly over the top of the casserole.
Bake uncovered for 30-35 minutes, until the casserole is bubbly around the edges and the topping is golden brown. Let it rest for 5 minutes before serving.