Spread 1/2 tablespoon mayonnaise over each of 8 bread slices. Layer each of 4 bread slices with 1 cheese slice, 1/2 cup arugula, 2 ounces chicken, a second cheese slice, and a bread slice.
. Heat a large nonstick skillet over medium heat. Coat sandwiches with cooking spray; add to pan. Cook 5 to 7 minutes on each side or until cheese melts and bread is toasted.