Place thawed spinach in a clean kitchen towel and wring firmly over the sink, working in sections, until no water drips from the cloth and the spinach is completely dry.
Preheat the oven to 190°C (375°F). Beat the softened cream cheese in a large mixing bowl for 1 minute until smooth and lump-free. Add the sour cream, mayonnaise, garlic, salt, pepper, and onion powder. Mix until evenly combined.
Fold in the squeezed spinach and chopped artichoke hearts with a silicone spatula, keeping artichoke pieces intact. Stir in the Parmesan and 40g (half) of the shredded mozzarella.
Spread the mixture evenly into a 20x20cm (8x8 inch) baking dish. Scatter the remaining 40g of mozzarella over the surface.
Bake for 22 to 25 minutes until the edges are bubbling and the mozzarella top shows golden-brown spots. For more colour, broil for 2 to 3 minutes and watch closely.
Rest for 5 minutes before serving.
