Whisk the vinegar and oil in a bowl, season. Add the celery, spinach and tarragon.
Mix the meat with the mustard. Heat the clarified butter in a frying pan. Fry the meat for approx. 8 mins., stirring occasionally, season. Add the rhubarb and sugar, cook for approx. 2 mins. Mix the chicken and rhubarb in with the salad.