Mix sour cream, mayo, cream cheese, 1 cup Parmesan cheese, and garlic until smooth. Stir in artichoke hearts and spinach.
Place bread, cut sides up, onto a baking sheet lined with parchment paper. Spread the spinach artichoke mixture evenly over the bread slices. Top with mozzarella cheese.
Bake for 10 minutes, broiling for the last minute.