Season
chicken with salt and pepper. Heat oil in a large, wide
saucepan and brown chicken on both sides. Remove chicken
from pan.
Add
onion, sweet potato, mushrooms and sage to the pan.
Cook and stir about 5 minutes until the onion is softenend.
Add up to 1/4 cup of water if required to keep from
sticking.
Sprinkle
flour over vegetables and cook and stir for 1 minute.
Stir in stock, vinegar and mustard, and bring to a boil.
Return
chicken to the pan. Cover and simmer for 20 minutes,
until chicken and sweet potatoes are cooked.