Set an oven rack to the middle position. Preheat to 400ºF (204ºC).
Lightly grease an 8-inch square baking dish with cooking spray or olive oil.
Place frozen spinach in a bowl and defrost at medium power for about 4 minutes, stirring halfway through. The leaves should be thawed but not hot.
Use hands to squeeze the excess water from the defrosted spinach.
In a medium bowl, use a spatula to mix together the drained spinach, chopped artichokes, cream cheese, mayonnaise, sour cream, 1 cup of mozzarella, ½ cup parmesan cheese, garlic, salt, and pepper until well combined.
Transfer spinach and artichoke mixture to greased baking dish. Spread the mixture evenly in the pan. Sprinkle ½ cup mozzarella cheese and ¼ cup parmesan cheese evenly on top.
Bake until cheese is lightly browned and bubbling around the edges, 20 to 25 minutes.
Served spinach artichoke dip with chips, sliced baguettes, or crackers.
Reheat: Preheat oven to 325ºF (163ºC). Bake dip until warmed throughout and cheese is melted, about 20 to 25 minutes.