Put lentils and chicken stock in a medium saucepan over medium-high heat; bring to a boil. Reduce the heat and cook at a slow boil until the lentils are softened but still firm to the bite, 15 to 20 minutes. Drain the lentils, transfer to a large bowl, and cool to room temperature.
Mince tarragon leaves and add them, with the turkey breast, red onion, celery and parsley to the cooked lentils. Mix gently.
Whisk the remaining ingredients together in a small bowl; add to the lentil mixture. Mix gently but thoroughly. If desired, chill before serving.