Preheat oven to 375°F (190°C). Grease a 2-quart baking dish.
Squeeze thawed spinach in a clean kitchen towel to remove excess water.
In a large bowl, combine cream cheese, mayonnaise, and sour cream until smooth.
Mix in minced garlic, salt, pepper, and cayenne if using.
Stir in squeezed spinach and chopped artichoke hearts.
Add 3/4 cup mozzarella and 1/2 cup Parmesan cheese, mixing until combined.
Transfer mixture to prepared baking dish and spread evenly.
Top with remaining mozzarella and Parmesan cheese.
Bake for 25-30 minutes until hot, bubbly, and golden on top.
Let cool for 5 minutes before serving.
Serve hot with tortilla chips and toasted baguette slices.
