Trim tops and bottoms off 2 oranges. Using a small, sharp knife, cut off peel, removing as much of the bitter white pith as possible. Working over a bowl, cut in between membranes to release orange segments. Squeeze juice out of membrane and squeeze remaining orange to get 1/2 cup juice. Set aside segments and juice separately.
In a large, resealable plastic bag, combine flour, 1/4 tsp. salt and 1/8 tsp. pepper. Add chicken breasts and shake to coat; shake off excess flour.
In a large nonstick skillet, heat oil over medium-high heat. Add chicken, skinned side down, and cook for 4 minutes. Turn and cook until chicken is just firm to the touch, about 4 minutes longer. Transfer chicken breasts to a plate.
Add onion and wine to pan and cook, stirring, until wine is evaporated. Add broth and reserved juice and boil, stirring occasionally, until sauce is reduced by half and onion is tender, about 5 minutes. Stir in mustard and remaining 1/4 tsp. salt and 1/8 tsp. pepper.
Return chicken to skillet and cook, turning to coat, for 1 minute. Transfer to plates. Add reserved orange segments to sauce, then spoon sauce over chicken. Sprinkle parsley on top.