Preheat your oven to 350 degrees F. Coat the inside of a 9-inch x 9-inch baking dish with non-stick cooking spray. Set aside.
Add the artichoke hearts, spinach, cream cheese, 2 cups of Mozzarella cheese, sour cream, lemon juice, lemon zest, Dijon mustard, salt, garlic powder, and thyme to a large mixing bowl. Mix until well combined with a hand mixer. (It's a lot easier than mixing by hand.)
Transfer the dip mixture into the prepared baking dish and top with the remaining ¾ cup of mozzarella cheese.
Bake for 25-30 minutes or until the cheese has melted and the edges of the dip start to turn golden brown.
Remove from the oven and top with some red pepper flakes, if desired. Serve warm with crackers, chips, or bread for dipping.
Notes
To cut down on dishes, you can mix the dip ingredients together in the square baking dish if desired. (I'm just prone to knocking things over the edge so I like to mix it in a bowl first.)