Cut the ciabatta bread in half.
Season the chicken breast with salt and pepper, grill until golden brown, then slice thinly.
Wash tomato and iceberg lettuce thoroughly and slice them.
Slice the avocado thinly.
Spread mustard sauce on one side of the ciabatta.
Layer the chicken breast, tomato, lettuce, and avocado in order on the ciabatta.
Cover with the other piece of ciabatta bread and cut in half.