Split each chicken breast in half lengthwise to form 2 cutlets. Heat a large skillet over medium-high heat. Coat the pan with cooking spray. Sprinkle the chicken evenly with pepper. Add the chicken to the pan; cook 4 minutes on each side or until done.
Remove the chicken from the pan. Wrap 1 prosciutto slice around each chicken cutlet.
Return the pan to medium heat. Coat the pan with cooking spray. Spread one side of each bread slice evenly with 1/2 tsp butter. Place 4 bread slices, buttered side down, in the pan. Top each bread slice in the pan with 2 tbsp cheese; toast the bread for 2 minutes or until the underside is toasted and the cheese melts. Remove the bread from the pan. Recoat the pan with cooking spray. Place the remaining 4 bread slices, buttered side down, in the pan. Toast 2 minutes or until toasted. Remove from the pan.
Top each cheese-topped bread slice with 1 chicken cutlet. Spread 1 1/2 tsp mustard over the untoasted side of each remaining bread slice.
Combine arugula, shallots, and juice in a bowl; toss. Divide the mixture evenly among the sandwiches; top each sandwich with 1 bread slice, mustard side down.