Prepare the Spinach: Thaw the frozen spinach completely and place it in a clean kitchen towel or cheesecloth. Squeeze out as much liquid as possible. This helps keep the dip thick and creamy rather than watery.
Prepare the Artichokes: Drain the artichoke hearts thoroughly and chop them into small pieces. If they are packed in brine, give them a quick rinse and pat them dry before chopping.
Mix the Creamy Base: In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Stir until smooth and creamy, with no large lumps remaining.
Add the Seasonings: Mix in the minced garlic, onion powder, Italian seasoning, black pepper, and salt. Stir until the seasonings are evenly distributed throughout the cream cheese mixture.
Fold in the Vegetables and Cheese: Add the spinach, chopped artichoke hearts, ¾ cup mozzarella, and Parmesan. Fold everything together until well combined. Reserve the remaining mozzarella for the topping.
Transfer to the Baking Dish: Preheat the oven to 375°F (190°C). Lightly grease an 8-inch baking dish with olive oil or butter. Spread the spinach and artichoke mixture evenly into the dish and sprinkle the reserved mozzarella over the top.
Bake Until Bubbly: Bake uncovered for 25–30 minutes, until the dip is hot throughout, bubbling around the edges, and lightly golden on top. For a more browned finish, broil for 1–2 minutes, watching carefully.
Rest and Serve: Let the dip rest for about 5 minutes before serving. Garnish with a little extra Parmesan or chopped parsley and serve warm with bread, crackers, or fresh vegetables.