Sprinkle in the flour, stir to coat the onions, then add any juices from the sausage plate along with the cider vinegar and stock. Bring everything in the pan to the boil before reducing the heat ...
Add the remaining oil and onion and cook together for 3 minutes more, or until the onion is softened and the sausage pieces are lightly browned, stirring regularly. Add a little more oil if needed.