In a large bowl, combine mozzarella, sour cream, mayo, 1/4 cup parmesan cheese, mustard, and pepper. Stir in artichoke hearts, spinach, and onion. Spread mixture evenly in a 9 inch pie plate. Sprinkle the top with the remaining parmesan cheese.

Bake uncovered in 350°F oven about 15 minutes or until heated through. Serve with crackers or pita. If you make it ahead of time, uncover and bake for approximately 25 minutes.