Cut the onion bagel in half.
Wash the arugula thoroughly and remove excess water.
Thinly slice tomato and onion.
Lightly toast the cut sides of the bagel in a pan.
Spread cream cheese on the bottom half of the bagel.
Place sliced tomato, onion, and arugula on top of the cream cheese.
Add chicken breast if desired.
Cover with the top half of the bagel to complete the sandwich.