Add the 2 tablespoons of vegetable oil to the skillet and add the onion. Cook over moderate heat, stirring a few times, until softened, about 10 minutes. Add the barley and stir well. Add 1 cup of the hot broth and cook over moderate heat, stirring often, until most of the broth has been absorbed. Continue adding more broth, 1 cup at a time, and stirring often between additions. The soup is finished when all of the broth has been added and the barley is just tender, about 25 minutes. Add the Scotch whisky and then stir in the butter. Season the soup with salt and pepper.