Prepare the chicken breasts by seasoning them with salt and pepper.
Cook the chicken on a grill or skillet over medium heat for about 6-7 minutes per side, or until fully cooked.
While the chicken is cooking, slice the tomatoes and mozzarella cheese.
Once cooked, let the chicken rest for a few minutes before slicing it.
Spread a tablespoon of pesto on one side of each ciabatta slice.
Layer the sliced chicken, mozzarella, and tomatoes on top of the pesto on two slices, then top with the remaining ciabatta slices, pesto-side down.
Brush the outsides of the sandwiches with olive oil.
Heat a skillet or panini press and grill the sandwiches for about 3-4 minutes on each side.
Remove from heat, cut in half if desired, and serve immediately.