Thaw the frozen spinach completely and squeeze out as much excess moisture as possible using a clean kitchen towel or paper towels. Drain and roughly chop the canned artichoke hearts. This step is crucial to prevent the dip from becoming watery.
In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Stir until smooth and creamy without lumps.
Mix in the minced garlic, onion powder, black pepper, and salt. Then fold in the shredded mozzarella and grated Parmesan cheese, reserving a small handful of mozzarella for topping if desired.
Gently fold the drained spinach and chopped artichokes into the creamy mixture until evenly distributed.
Spread the dip evenly into your greased baking dish. Sprinkle any reserved mozzarella on top for a golden crust. Bake at 375°F (190°C) for about 25-30 minutes, or until bubbly and lightly golden on top.
Let the dip cool for a few minutes before serving. Pair it with pita chips, sliced baguette, or fresh veggies for dipping.