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.
Serve warm.