This dish is quite spicy. For milder flavor, use only 1 chipotle pepper.
Blend the canned tomatoes and chipotle peppers in blender until smooth. Add to browned chicken in skillet along with the onions; continue as directed.
If your chicken breasts are larger than 4 oz. each, they will take longer to cook. Increase the cook time as necessary to be sure the chicken is done (165ºF).