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).

Prepare the vegetables: Ensure the frozen spinach is fully thawed. Using a clean kitchen towel or several layers of paper towels, squeeze the spinach vigorously until all excess moisture is removed. This step is crucial to prevent a watery dip.

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 fully smooth and uniform.

Season the base: Add the minced garlic, onion powder, black pepper, salt, and optional red pepper flakes to the cream cheese mixture. Stir thoroughly to distribute the seasonings evenly.

Add in the solids: Fold in the squeezed spinach, the chopped artichoke hearts, Parmesan cheese, Monterey Jack cheese (if using), and 3/4 cup of the shredded Mozzarella cheese. Gently mix until everything is just combined.

Transfer 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 for a golden crust.

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 for 5 to 10 minutes before serving. Serve hot with dippers such as tortilla chips, pita chips, crostini, or fresh vegetables.