Place chicken in a single layer in a frying pan. Cover with stock. Remove leaves from tarragon sprigs; set aside. Lightly crush stalks with a rolling pin, then add to pan. Using a vegetable peeler, remove a wide strip of rind from lemon. Add to pan with peppercorns.
Place frypan over moderate heat and bring stock to a boil. Reduce heat to a simmer, cover pan and cook 15 minutes, or until chicken is cooked through. Remove chicken from pan with a slotted spoon; leave to cool. Strain stock and reserve. Discard stalks, rind and peppercorns.
To make dressing, whisk tahini with 4 tablespoons reserved stock. Add another 1 to 2 tablespoons if mixture is too thick. Squeeze juice from lemon; stir into dressing. Chop enough tarragon leaves to make 1 tablespoon. Add to dressing. Add salt and pepper to taste.
Slice witlof diagonally into narrow pieces. Place on a large serving platter with spinach leaves. Add orange segments and toasted almonds. Slice chicken into wide strips and add to salad. Spoon on dressing.