Preheat over to 400F. Drain as much liquid from thawed spinach as possible. Stir together spinach, artichokes, cream cheese, mayonnaise, mozzarella, parmesan, garlic powder, black pepper and crushed red pepper in large bowl until thoroughly combined. I used a hand mixer to make everything smooth. You could also mix everything in a food processor. If you really like to do things by hand, you can do that to. Make sure the cream cheese is softened.
Pour dip into 8 inch square glass baking dish.
Stir together the panko and the melted butter. Sprinkle evenly over dip. (If you want gluten free option you can skip this step entirely. We actually served this in a bread bowl, so I think the prettiness of the panko was lost.)
Bake for 25 to 30 minutes until top is bubbly and golden brown. Let cool 5 minutes. Serve with your favorite pita chips, crackers, sliced bread or with vegetables like broccoli.