Core the tomatoes and slice each tomato and each ball of mozzarella into 1/4-inch thick slices.
Place the sliced ciabatta loaves on a work surface, and spread each half evenly with 1/8 cup pesto. Place 4 slices of tomato and 3 slices mozzarella side by side on the top of each half loaf and sprinkle with salt and pepper. Place 4 basil leaves on top of each sandwich.
Slice each half loaf crosswise into 4 serving portions, making sure that each piece has a slice of tomato, mozzarella and a basil leaf.