Combine marinade ingredients in a bowl.
Pour marinade into resealable plastic bag.
Cut chicken breasts in half, place in bag, and seal the bag securely.
Turn bag a few times so all portions of chicken are coated by marinade.
Refrigerate and marinate for 1 hour.
Drain, discard marinade.
Separate skin from chicken.
Toss mushrooms with sage.
Slide mushrooms under the skin.
Preheat stovetop grill.
Cook chicken over medium-high heat, skin side down, for about 5 minutes, turning once.
Chicken will be slightly firm to the touch and juices will run clear when chicken is cut with a knife.
Remove chicken to serving dish