1. Preheat oven to 325°F. In a 9 x 13-inch baking dish, toss squash cubes, onions, pecans, sage, vinegar, nutmeg and salt. Nestle chicken breasts into squash mixture. Cover with parchment paper then with foil, sealing well. Bake for 45 minutes.
2. Meanwhile, in a small bowl, whisk together mustard, honey and Worcestershire sauce.
3. Remove baking dish from oven. Increase oven temperature to 400°F. Uncover baking dish and spread mustard mixture over chicken breasts. Drizzle apple cider over vegetables; gently stir.
4. Continue baking until chicken is cooked through and an instant-read thermometer inserted into thickest part of one breast registers 165°F, about 30 minutes. Let chicken stand in baking dish for 5 minutes. Transfer chicken to a cutting board; slice and serve with vegetables.
Tip: This recipe works best with large boneless, skinless chicken breasts. They stay juicier as they roast.