Combine mayonnaise, sour cream, Parmesan cheese, the 1/2 cup mozzarella cheese, mustard, and pepper. Stir in artichoke hearts, spinach leaves, and onion.
Transfer mixture to a 1-quart casserole. Cover; chill in refrigerator for at least 3 hours or up to 24 hours.
Bake in a 350 degree F oven for 50 to 60 minutes or until heated through. Top with 1/4 cup mozzarella cheese. Bake for 5 minutes or until cheese melts.
Serve with pita, bread or crackers. Makes 24 appetizer servings.