Preheat oven to 375F.
If you like, pulse the artichoke hearts and spinach with the sour cream or yogurt in a food processor until roughly chopped. In a bowl, mash together the cream cheese, mayo, Parmesan cheese, garlic and salt and pepper until well blended. Stir in the artichoke-spinach mixture and spread in a shallow baking dish. Sprinkle with grated mozzarella or more Parmesan cheese. (The photo above is just Parmesan.)
Bake until heated through and bubbly, about 25 minutes.
Serve warm with tortilla chips or sliced soft, crusty baguette.