How to Make It
Preheat the oven to 350°. In a medium saucepan, cover the chicken breasts with water. Add the bay leaf and bring to a simmer. Reduce the heat to low and poach the chicken until white throughout, about 10 minutes. Discard the bay leaf. Transfer the chicken to a work surface and let cool slightly, then cut into 1/2-inch dice.
Meanwhile, in a pie plate, toast the walnuts for about 7 minutes, or until golden brown. Let the walnuts cool, then coarsely chop them.
In a large bowl, whisk the mayonnaise with the mustard, tarragon, chives and lemon juice. Add the chicken, walnuts and celery, season with salt and pepper and toss to coat thoroughly.