Heat 1/4 cup butter and the vegetable oil in a frying pan and brown the chicken breasts. Season with salt and pepper. Cover, reduce the heat and cook for 10 minutes. Remove the chicken from the frying pan and cut into strips. Keep warm. Discard the butter and oil from the frying pan. Pour the vinegar into the frying pan and simmer for 1-2 minutes, scraping the bottom of the pan with a wooden spoon to dislodge any attached particles. Add the chicken stock to the vinegar and raise the heat to a boil. Boil hard until reduced by three-quarters. Remove the pan from the heat and whisk in the butter, little by little, until sauce thickens. Season with salt and pepper. Toss together the watercress, curly endive and red leaf lettuce in a salad bowl. Place the chicken strips on top and pour in the sauce. Season with salt and pepper and toss well before serving.
