What are you trying to achieve? Onions are usually sweated, then other ingredients added that will protect the onion from over cooking, often including liquids. For a seared/sauteed chicken breast, this is usually done with only chicken in the pan. While the chicken is resting after cooking, you might build a pan sauce with onions. In a stir fry, the chicken is usually cooked first, removed, then onions cooked, then everything combined and sauced.
8/29/14 at 1:38pm