Preheat your oven to 375°F (190°C). Lightly grease a medium-sized baking dish (an 8x8 inch square dish or a small 1-quart casserole dish works well).

Remove all excess moisture from the spinach: Using a clean kitchen towel or paper towels, squeeze the fully thawed spinach vigorously until it is bone-dry. This step is crucial to prevent the dip from becoming watery.

Drain the artichoke hearts and roughly chop them into small pieces.

In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Use a hand mixer or a sturdy spoon to beat the mixture until it is smooth and uniform.

Add the minced garlic, onion powder, black pepper, and salt to the cream cheese mixture. Stir thoroughly to distribute the seasonings evenly.

Add the squeezed, dry spinach and the chopped artichoke hearts to the creamy base.

Fold in the grated Parmesan cheese, shredded Monterey Jack cheese (if using), and 3/4 cup of the shredded Mozzarella cheese. Reserve the remaining 1/4 cup of Mozzarella for the topping. Stir gently until all ingredients are just combined.

Spoon the prepared dip mixture into the lightly greased baking dish, spreading it out into an even layer.

Sprinkle the reserved 1/4 cup of Mozzarella cheese over the top of the dip.

Bake for 20 to 25 minutes, or until the edges are bubbling vigorously and the cheese topping is fully melted and lightly golden brown.

Remove the dip from the oven and allow it to cool and set up slightly for 5 to 10 minutes before serving. This prevents the dip from being too runny and prevents burns.

Serve hot with tortilla chips, pita chips, crostini, or vegetable sticks.