Preheat oven to 350°F and grease a 2-quart casserole dish.
In a large mixing bowl, combine cream cheese, sour cream, mozzarella, Parmesan, garlic, onion powder, salt, and pepper until well blended.
Fold in the drained spinach and chopped artichoke hearts until evenly distributed throughout the cream cheese mixture.
Spread the mixture evenly into the prepared casserole dish.
In a small bowl, toss panko breadcrumbs with melted butter until well coated, then sprinkle evenly over the top of the casserole.
Bake for 28-30 minutes until the casserole is hot and bubbly and the breadcrumb topping is golden brown.
Let cool for 5 minutes before serving.