Place mustard in a shallow dish or bowl; place onions in a second shallow dish or bowl. Dredge chicken in mustard to coat both sides, then dredge in onions. Place coated chicken in a lightly greased 9x13 inch baking dish.
Bake at 375 degrees F (190 degrees C) for 50 to 60 minutes or until chicken is cooked through and juices run clear.
