Cook hot & spicy filet according to package directions.
While chicken is cooking, place a saute pan on medium heat and add sliced onion. Cook for 10-15 minutes, stirring often until caramelized and golden brown. If the onion is beginning to brown too much or unevenly, add a little water to the pan.
To assemble the sandwich, spread arugula evenly around the bottom of the bun, top the arugula with the fully cooked chicken breast. Follow with the onions, cheese and top off the roll.