Add the onion, carrot, and apple to the pan. Cook over moderate heat, covered, for 5 minutes. Stir in the sauerkraut, bacon, broth, the remaining 1/4 teaspoon salt, the mustard, juniper berries, and bay leaf. Arrange the chicken in an even layer on top. Bring to a simmer, reduce the heat, and cook, covered, until the chicken is just done, about 25 minutes.