NOTE: This recipe requires 2 to 4 hours of time for the chicken to marinate!
In a large bowl whisk together the apple cider, Dijon mustard, ground sage, and salt.
If the chicken breasts are very thick, either filet them or lightly pound with a mallet until even in thickness.
Add the chicken breasts to the marinade, ensuring all the chicken is covered in the mixture. Cover the bowl with plastic wrap and allow to marinate in the refrigerator for 2 to 4 hours.
Preheat oven to 350F. Place the chicken breasts in a single layer in a 13x9 baking dish. Arrange the quartered apples and shallot around the chicken before laying the rosemary sprigs on top of the breasts. Drizzle the olive oil over the entire dish (using your hands to ensure the oil is on each piece of apple and onion).
Bake the chicken for 30 to 40 minutes or until a meat thermometer reads 165F when inserted into the thickest part of the breasts. NOTE: The thickness of the breasts will determine how long to cook. Thinner breasts, shorter cook time. Thicker breasts, longer cook time.
Remove the rosemary sprigs and allow the chicken to rest for 10 minutes before serving.