Toast the ciabatta slices until golden brown and crispy, then set aside to cool slightly—this helps prevent sogginess.
Arrange slices of fresh mozzarella over the pesto on two of the bread slices, pressing gently to let the cheese settle in.
Place the tomato slices over the mozzarella, slightly overlapping them to create a colorful, fresh layer.
Carefully top each sandwich with the remaining bread slices, pesto side down, then gently press to compact the layers.
If desired, slice each sandwich diagonally and serve immediately for best texture and flavor.