Reduce the heat to medium. Add the onion and thyme to the same pan and cook, stirring, for 5 mins, or until softened. Sprinkle the flour over the onion and cook, stirring, for 1 min, or until foaming. Gradually add the stock mixture, stirring constantly to prevent lumps from forming. Bring to a simmer. Then add the chicken and any resting juices to the gravy. Reduce the heat to low and cook, turning the chicken halfway, for 4-5 mins, until the gravy is thickened and the chicken is cooked through. Stir in balsamic vinegar.