Poached Chicken and Walnut Salad

2 whole chicken breasts (with skin on and bone in)
1 celery stalk
1 onion, sliced
6 peppercorns
chicken broth
3 tbsp. extra virgin olive oil
3 tbsp. balsamic vinegar
1/4 cup walnuts
mixed greens: watercress, arugula, red leaf lettuce, radicchio

Place chicken breasts in pan or skillet. Add celery, onion and peppercorns. Add enough broth to cover chicken. Bring to a boil, then reduce heat to medium-low. Cook until just tender, about 20 minutes. Cool chicken and remove skin. Pull chicken off bone in strips. In a saucepan, combine olive oil, vinegar and walnuts. Cook until somewhat thickened and liquid is reduced. Wash greens and tear into pieces. Place greens on four salad dishes, then arrange chicken on each plate of greens. Pour oil-vinegar mixture on top.