Put chicken in a large skillet not too deep that has simmering salted water. Cover and cook at medium-low heat for about 12 minutes or until the chicken is not pink inside (flip it at mid-cooking). Put the chicken in a plate and let cool in the fridge. Cut the cold chicken in dice.
In a large bowl, mix celery, mayonnaise, dill, Dijon mustard, zest and lemon juice, salt and pepper. Add chicken and stir to coat well. (You can do the chicken garnish in advance and cover it. It will be good up to 4 hours in the fridge.).
Put 1/4 cup of watercress in the center of each half-tortillas. Cover with 3/4 cup of the chicken garnish. Bringing one of the point towards the center, roll half-tortillas on the garnish in a way to form a horn. Envelop the base with waxed paper or fix it with long tooth-pick.