Chicken and Brussels Sprouts with Apple Cider Sauce

Name: Chicken and Brussels Sprouts with Apple Cider Sauce
Ingredients: 1 pound Brussels sprouts, trimmed and halved or quartered if large; 2 sweet apples (such as Gala), cut into wedges; 1 red onion, cut into wedges (root left intact); 2 small sprigs rosemary, plus 1 teaspoon finely chopped leaves; 2 tablespoons extra-virgin olive oil; Kosher salt and freshly ground pepper; 2 tablespoons unsalted butter; 4 skinless, boneless chicken breasts (about 8 ounces each); 2/3 cup apple cider; 1 teaspoon apple cider vinegar