Preheat your oven to 350F and move the rack to the top third of the oven.
Thaw the spinach, then drain it and squeeze out as much of the excess water as you can. To measure it out, loosely pack the measuring cup full.
Add the cream cheese to a medium prep bowl and microwave about 30 seconds or until the cream cheese is softened.
To the same bowl, add in the parmesan cheese, some salt & pepper, the spinach, and the artichoke hearts.
In a skillet over medium heat, melt the butter. Add in the onion and sauté until softened and lightly browned (about 5-7 minutes), then stir in the garlic. Transfer the onions/garlic mixture to the prep bowl, and then give it a good stir.
Pour the dip mixture into a 1-quart baking dish. Sprinkle the mozzarella over top. Bake, uncovered, for 25 minutes or until hot and bubbly, then broil at the end to brown the cheese a bit more (watch it carefully).
Serve hot with tortilla chips, baguette crisps, veggies, etc.