Poach the chicken: Place the chicken breasts in a saucepan and cover with the stock. Add the bay leaf and peppercorns. Bring to a gentle simmer over a medium heat and cook for 15 to 20 minutes until the internal temperature reaches 74C (165F). Do not let it boil. Remove the pan from the heat and leave the chicken to cool completely in the liquid.
Make the dressing: Whisk together the mayonnaise, Greek yoghurt, Dijon mustard and lemon juice in a large bowl. Season with salt and pepper. Taste and adjust the lemon or mustard to your liking.
Shred and mix: Remove the cooled chicken from the stock and shred or dice it into bite-sized pieces. Pat dry with kitchen paper. Add the chicken, celery, spring onions, tarragon, parsley and nuts to the dressing. Fold gently until everything is evenly coated.
Chill and serve: Cover and refrigerate for at least 30 minutes to let the flavours come together. Serve on toasted sourdough, in a wrap, or spooned into little gem lettuce cups.
