While bread toasts, whisk together yogurt, water, mayonnaise, Parmesan, mustard, salt, and pepper in a large bowl. Cut 5 inches from the top of each romaine lettuce heart; set aside 8 of the largest leaf tops for lettuce cups. (Save remaining leaves for another use or discard.) Chop bottom section of each romaine lettuce heart into bite-size pieces. Add chopped lettuce, chicken, and cooled bread to bowl with dressing; toss to coat. Divide evenly among 8 reserved lettuce cups.