Heat the oil in a 12-inch skillet and brown the chicken over medium-high heat. Drain. Add the corn, peas and carrots. Blend the soup mix with the water. Add to the skillet along with the sherry and mustard. Bring to a boil, then simmer, covered, for 20 minutes.
Remove the chicken and vegetables from the skillet to a serving platter and keep warm; reserve the liquid. Boil the liquid over high heat for 10 minutes. Remove from the heat and stir in the flour blended with the sour cream. Return to the heat and bring to a boil; reduce the heat and simmer, stirring constantly, until the sauce thickens. Pour the sauce over the chicken and vegetables and serve.
