Place the chicken on a clean cutting surface.
With the tip of a sharp knife, slit each breast open without cutting through the entire breast, to create a pocket.
In a medium bowl, toss together the stuffing mix, onion, sage, and rosemary.
Stuff one-fourth of the dressing in each chicken breast and press the edges of the chicken breast together to close the pocket.
A small bit ofstuffing may spill out slightly during grilling, but this will not affect the outcome of the chicken.
Lightly coat the grill with cooking spray and preheat for 5 minutes.
Grill the stuffed chicken for 7-9 minutes, or until the chicken is fully cooked and the stuffing is warm.
Scoop up any excess stuffing with the plastic spatula and serve with the chicken.