This is an ultra-simple and flexible recipe. But because of its simplicity, it's essential that you seek out only the freshest, best quality ingredients. If you can't find the red butter lettuce I used, try French Crisp, Red Oakleaf, or Lollo Rossa. Any of those varieties will be delicious, and will provide a crisp and delicate contrast to the creamy mozzarella and the juicy tomatoes.