Cider-Braised Chicken

1 tablespoon pure or extra-virgin olive oil
4 whole chicken legs or bone-in, skin-on chicken breasts
salt and freshly ground black pepper
1 large Spanish onions, halved crosswise and thinly sliced
4 sage leaves
2 bay leaves
1/4 teaspoon crushed red pepper
2 1/2 cups apple cider
1/4 cup cider vinegar
3 cups chicken stock or low-sodium broth
1 tablespoon chopped flat-leaf parsley